it,fl  an 


ECTIC 
PHYSI 

GEOGRAPHY 

HINMAN 


BERKELEY 

LIBRARY 

UNIVERSITY  OF 
CALIFORNIA 


x-t, 


&M&* 


Come  and  see  the  works  of  God. 

—Psalm  lxvi:  5. 


Old  Faithful  Geyser  in  Eruption, 
[frontispiece.)  -» 


Yellowstone  Park,  Wyoming. 


ECLECTIC  /^-^ 


PHYSICAL    GEOGRAPHY 


BY 


RUSSELL  HINMAN 


NEW  YORK     •:•     CINCINNATI     .;.     CHICAGO 

AMERICAN  BOOK  COMPANY 


The  Eclectic  Geographies. 

Two- Book  Series. 

ELEMENTARY  GEOGRAPHY  $0.55 
COMPLETE  GEOGRAPHY  $1.20 

THESE  GEOGRAPHIES  ARE  UNRIVALLED  in 
simplicity  and  precision  of  text,  and  scientific  accuracy 
of  maps.     Descriptive  price-list  on  application. 


EARTH 
SCIENCES 

UBlAriY 


•  •  •  j •  « 


2  *j  *********  *     j    * 

*.*«  >••_••••?    *  *v. 


Copyright,  1888,  by  Van  Antwerp,  Bragg  &  Co. 
Copyright,  1897,  by  American  Book  Company. 


EC  PHVS.  GEOO. 
E-     37 


GEOGRAPHY  DEPT. 


PREFACE 


The  aim  of  this  book  is  to  indicate  briefly  what  we  know  or  sur- 
mise  concerning  the  proximate  causes  of  the  more  common  and 
familiar  phenomena  observed  at  the  earth's  surface.  Even  thus  re- 
stricted, the  field  of  inquiry  encroaches  to  a  greater  or  less  extent 
upon  the  domains  of  all  the  branches  of  science. 

Since  the  study  of  Physical  Geography  precedes  that  of  the 
sciences  in  most  of  our  schools,  it  has  been  thought  advisable  to. 
present,  in  the  form  of  an  introductory  chapter,  a  condensed  state- 
ment of  the  more  important  and  fundamental  scientific  conceptions' 
regarding  the  properties  and  phenomena  of  matter  and  energy, 
such  as  inertia,  gravitation,  cohesion,  affinity,  and  heat,  light,  mag- 
netism, and  electricity.  This  chapter  may  be  studied  or  simply  readJ 
at  the  discretion  of  the  teacher.  » 

The  order  of  treatment  of  the  different  parts  of  the  subject  proper 
is  that  which  seems  most  natural  and  rational.  After  describing 
the  relations  of  the  planet  to  the  solar  system,  its  movements  and 
their  effects,  the  atmosphere  is  at  once  considered,  not  only  be- 
cause it  forms  the  true  outer  layer  and  envelope  of  the  earth,  but 
because  its  action  is  the  proximate  cause  both  of  all  details  in  the 
relief  of  the  land,  and  of "t&e.  more  conspicuous  phenomena  of 
the  sea.  The  sea  is  next  discussed,  since  it  forms  an  intermediate 
layer  between  the  atmosphere  and  three  fourths  of  the  earth's  solid 
surface,  and  since  the  peculiarities  in  the  relative  position,  compo- 
sition, and  relief  of  the  land  masses  can  be  appreciated  only  after 
some  acquaintance  with  the  depth  and  character  of  the  bottom  in 
different  regions  of  the  sea.  In  the  treatment  of  the  land,  which 
then  follows,  the  methods  by  which  its  surface  contour  is  constantly 
modified  by  atmospheric  agencies  are  explained  at  greater  length 
than  is  usual  in  current  text-books,  while  the  influence  of  subter- 
ranean agencies  in  changing  the  elevation  of  the  land  is  carefully 
considered.  Climate,  being  the  average  local  condition  of  the  atmos- 
phere, as  determined  largely  by  the  peculiarities  of  the  surface  upon 

(iii) 

990903 


IV  PREFACE. 

which  it  rests,  is  appropriately  treated  after  that  surface  has  been 
discussed,  and  fittingly  precedes  the  concluding  chapters  on  life. 
These  chapters  embrace  a  brief  description  of  the  more  conspicuous 
phenomena  of  the  organic  world.  The  close  dependence  of  plants 
and  animals  upon  their  inorganic  surroundings  and  upon  each  other 
is  pointed  out,  as  well  as  the  remarkable  series  of  facts  which  is  held 
by  many  scientists  to  indicate  that  all  organisms  are  of  kin. 

In  a  study  of  this  kind  it  is  well  to  remember  that,  with  all  the 
scientific  knowledge  of  the  nineteenth  century,  we  are  still  profoundly 
ignorant  of  the  ultimate  causes  of  things,  while  our  ideas  of  prox- 
imate causes  are  constantly  being  revised  and  changed  as  our 
acquaintance  with  nature  increases.  The  broadest  scientific  gener- 
alizations of  one  generation  are  apt  to  be  swept  away  or  greatly 
modified  by  the  next,  and  our  descendants  will  doubtless  regard 
the  science  of  to-day  much  as  we  regard  that  of  the  ancients.  Yet, 
notwithstanding  its  crudities  and  absurdities,  ancient  alchemy  grad- 
ually developed  into  modern  chemistry,  which  has  been  of  inesti- 
mable value  to  man ;  and  in  the  same  way,  the  more  perfect 
knowledge  of  the  future  is  to  be  acquired  only  through  familiarity 
with  the  imperfect  theories  of  to-day. 

The  maps  in  the  book  have  been  carefully  prepared  in  various 
projections,  each  adapted  to  portray  most  accurately  the  special 
feature  under  consideration,  and,  with  the  cuts  and  diagrams,  are 
inserted  to  illustrate,  and  not  simply  to  beautify  the  text. 

The  acknowledgments  of  the  author  are  due  to  the  United  States 
Geological  Survey,  Coast  and  Geodetic  Survey,  Signal  Service,  and 
Hydrographic  Bureau  for  maps  and  information  courteously  sup- 
plied. He  also  takes  this  opportunity  to  acknowledge  gratefully  the 
valuable  assistance  rendered  in  revising  the  proof-sheets  of  the 
introductory  chapter,  by  Prof.  T.  H.  Norton,  of  the  University  of 
Cincinnati;  of  the  chapters  on  the  atmosphere  and  climate,  by 
Prof.  Cleveland  Abbe,  of  the  U.  S.  Signal  Service,  and  Prof.  W.  M. 
Davis,  of  Harvard  College;  and  of  the  chapters  on  the  land  by 
Capt.  C.  E.  Dutton  of  the  U.  S.  Geological  Survey. 


CONTENTS. 


Introduction. — Some  General  Laws  of  Nature 

Part  I. — The  Earth  as  a  Planet. 


Chapter        I—  The  Solar  System    . 

"  II. — Movements  of  the  Earth 


PAGE 

7 


35 
43 


Part  II. — The  Atmosphere. 

Chapter     III. — Composition,  Weight,  and  Heat    . 
IV. — Moisture  of  the  Atmosphere   . 
V. — Movements  of  the  Atmosphere 
VI.—        "  "     "  "  {Continued) 


55 
66 

78 
90 


VII. — Luminous  Phenomena 100 


Part  III.— The  Sea. 

Chapter  VIII. — Depth,  Composition, and  Temperature    . 

IX.— Waves  and  Tides 

X. — Currents  and  Deposits     .... 


109 
122 
135 


Part  IV.— The  Land. 


Chapter        XI. — Divisions  of  the  Land 
XII. — Surface  of  the  Land 
XIII. — Structure  of  the  Land 
XIV. — Springs 
XV. — Streams 
XVI.— Work  of  Streams 
XVII. — Glaciers  and  Lakes 


149 
161 
180 

195 
205 
216 
231 


(v) 


VI 


CONTENTS. 


PAGE 

Chapter  XVIII. — Mountain  Structure  and  Land  Sculpture      .    248 

XIX. — Earthquakes 264 

"  ;  -         XX.— Volcanoes 278 


Part  V. — Weather  and  Climate. 
Chapter     XXI. — Weather  and  Climate  . 


293 


Part  VI. — Life. 

Chapter  XXII. — The  Various  Forms  of  Life 
XXIII.— Distribution  of  Life 
XXIV.— Man        .        ... 


•  313 
.  328 

•  35o 
Index 373 


List  of  Maps  and  Charts. 


Abnormal  Temperatures  of 

Isothermals,  United  States  . 

309 

the  World 

305 

Lake  Region  United  States  . 

239 

Biological  Regions 

334 

Magnetic  Meridians,  North- 

Continental Plateau     . 

152 

ern  Hemisphere 

3i 

Currents   and   Floating  Ice 

Malay  Archipelago 

156 

of  the  Sea 

138 

Map  Projections 

53-4 

Delta  of  Mississippi  and  Nile 

228 

Rain-fall   on    Land  Surface 

Depths  of  the  Sea       .         .  1 

12-3 

of  the  World    . 

76 

Drainage   Basin   of   Missis- 

Rain-fall of  United  States    . 

3°7 

sippi  River 

207 

Range  of  Annual  Tempera- 

Drainage Basin  of  the  Oceans 

209 

ture  of  World 

300 

Earthquakes  in  the  U.  S.  . 

267 

Storm  Frequency  and  Paths 

9i 

Glaciers  and  Glaciated  Re- 

Surface Africa  and  Australia 

175 

gions         .... 

238 

Surface  of  Euro-Asia   . 

173 

Isobars  and  Winds  Northern 

Surface  of  North  and  South 

Hemisphere 

87 

America 

167 

Isobars  and  Winds  Southern 

Tornado  Frequency  in   the 

Hemisphere 

85 

United  States     . 

98 

Isothermals,  Northern  Hemi- 

Vegetation regions  of  World 

332 

sphere       .... 

63 

Volcanic  regions  of  World  . 

289 

Isothermals,  Southern  Hemi- 

Weather   Chart   of   eastern 

sphere       .... 

65 

United  States     . 

296 

INTRODUCTION 


SOME  GENERAL  LAWS  OF  NATURE. 

Show  me  thy  ways,   O  Lord,  teach  me  thy  paths. — Psalm  xxv:   4. 

Laws  of  Nature. — Nothing  in  nature  is  permanent; 
every  thing  is  constantly  changing.  Day  changes  into  night, 
fair  weather  into  foul,  plants  and  animals  die  and  decay,  and 
even  the  solid  rocks  gradually  wear  away  into  soil  or  sand. 
These  changes  do  not  occur  by  chance,  but  each  is  the 
result  of  some  definite  cause,  and,  under  similar  circum- 
stances, precisely  the  same  effects  are  produced  by  the 
same  causes.  The  invariable  relations  between  causes  and 
resulting  effects  constitute  the  laws  of  nature. 

Physical  Geography  seeks  to  trace  the  operation  of 
the  laws  of  nature  upon  the  earth ;  upon  the  air,  the 
water,  and  the  land ;  upon  plants,  animals,  and  even  upon 
man. 

Matter  is  the  general  name  given  to  the  material  of 
which  any  body,  such  as  rock,  water,  or  air,  is  composed. 

Kinds  of  Matter.— The  number  of  substances  in  the 
world  is  almost  infinite ;  yet  most  of  them  are  compound 
substances,  and  can  be  broken  up  into  a  comparatively 
few  distinct  kinds  of  matter,  from  which  no  other  kind  can 
be  obtained.  These  are,  therefore,  called  simple  substances, 
or  elements. 

<7> 


8  PHYSICAL  GEOGRAPHY. 

Constitution  of  Matter. — All  matter  is  conceived  to 
be  built  up  of  minute  particles,  called  molecules.  A  mole- 
cule is  the  smallest  fragment  of  any  substance  that  can 
exist  by  itself.  A  molecule  of  a  compound  substance 
breaks  into  the  simple  substances  of  which  the  compound  is 
composed ;  and,  a  molecule  of  a  simple  substance  breaks 
iato  still  smaller  fragments,  called  atoms,  which  are  too  small 
to  exist  by  themselves,  but  large  enough  to  unite  with 
other  atom 5  cf  the  same  or  different  kinds  of  matter  to 
form  a  simple  or  a  compound  molecule. 

Molecules  are  too  small  to  be  visible  even  with  the  aid  of  the 
most  powerful  microscope.  Some  idea  of  their  extreme  smallness 
may  be  gathered  from  Sir  William  Thomson's  estimate.  He  says 
that  if  a  drop  of  water  were  magnified  to  the  size  of  the  earth,  its 
molecules,  so  magnified,  would  be  about  as  large  as  base-balls. 

Common  Properties  of  Matter. — All  matter  of  what- 
ever kind  is  indestructible,  impenetrable,  and  possesses 
inertia.  By  virtue  of  the  first  quality, 
it  is  absolutely  impossible  to  destroy 
a  single  atom  or  molecule  in  nature. 
A  substance  may  disappear,  but  new 
substances  are  always  formed  of  its 
constituent  atoms.  By  virtue  of  the 
second  quality,  two  particles  can  not 
occupy  the  same  space  at  the  same 
time.  A  nail  driven  into  a  board 
Fie-  «•  does  not  penetrate  the  molecules  of 

the  wood :  it  simply  forces  them  aside. 

Inertia. — All  matter  resists  being  set  in  motion,  and, 
when  moving,  resists  any  change  in  the  rate  or  direction 
of  its  motion.  Hence,  no  body  can  either  start  into  mo- 
tion or  stop  when  moving  unless  something  outside  of  itself 
pulls  or  pushes  it  powerfully  enough  to  overcome  this  re- 
sistance.    This  resistance  of  matter  is  called  its  inertia. 


SOME   GENERAL    LAWS   OF    NATURE.  9 

The  inertia  of  bodies  increases  with  the  amount  of  matter  they 
contain.  Thus,  if  two  balls  of  iron,  a  large  one  and  a  small  one,  be 
suspended  by  long  cords,  the  large  one  will  require  a  more  powerful 
pull  or  push  to  start  it  to  swinging  or  to  stop  it  than  the  small  one. 
If  a  ball  of  cork,  of  the  same  size  as  one  of  the  iron  balls  be  sus- 
pended, it  will  require  a  less  powerful  pull  or  push  to  start  or  stop  it 
than  the  iron  ball.  The  iron  ball,  therefore,  though  of  the  same 
size,  contains  a  greater  quantity,  or  mass,  of  matter,  and  possesses 
greater  inertia  than  the  cork  ball. 

Forces  of  Nature. — A  pull  or  a  push  of  any  kind  or 
strength  always  tends  to  overcome  inertia,  and  is  called  a 
force.  There  are  three  great  natural  forces  by  whose 
varying  action  the  few  elemental  substances  are  gathered 
and  held  together  into  the  infinite  variety  of  groups  or 
bodies  of  nature.  These  forces  are  Gravitation,  Cohesion, 
and  Chemical  Affinity. 

Gravitation  is  a  force  which  is  constantly  acting  upon 
all  matter  in  nature.  By  virtue  of  this  force,  each  mole- 
cule of  matter  tends  to  attract,  or  pull  toward  itself,  every 
other  molecule,  however  distant.  Since  each  molecule 
exercises  this  attraction  upon  others,  a  body  composed  of 
a  great  number  of  molecules,  or  of  a  large  mass  of  matter, 
exercises  a  stronger  attraction  than  bodies  of  fewer  mole- 
cules or  less  mass.  The  mass  of  the  whole  earth  is  so 
enormous  that  its  attraction  quite  overpowers  that  of  de- 
tached bodies  near  its  surface  ;  these,  therefore,  if  unsup- 
ported, yield  to  the  attraction  of  the  greater  mass,  and 
move  or  fall  toward  the  earth.  If  the  body  is  sup- 
ported, the  attraction  of  the  earth  causes  it  to  push  or 
press  against  its  support.  This  pressure  is  called  the 
weight  of  the  body. 

Some  bodies  do  not  fall,  but  ascend — as  smoke  or  a  balloon  in 
the  air,  or  a  cork  or  oil  under  water.  This  is  not  because  the  earth 
does  not  attract  them,  but  because  an  equal  bulk  of  the  air  or  water 
immediately  above  the  body  contains  a  greater  mass  of  matter,  and 
is,  therefore,  more  strongly  attracted  by  the  earth  than  the   body 


IO 


PHYSICAL    GEOGRAPHY. 


itself.  The  body  and  the  greater  mass  of  air  or  water  above  it  con- 
sequently exchange  places — the  greater  mass  sinking,  and  forcing 
the  smaller  mass  to  rise.  When  two  bodies  are  weighed  in  the  same 
place  and  under  similar  conditions,  the  heavier  always  contains  the 
greater  mass  of  matter  even  if  it  is  much  the  smaller  body.  Thus, 
i  cubic  foot  of  rock,  2  cubic  feet  of  water,  8  cubic  feet  of  cork,  and 
1,600  cubic  feet  of  air  have  about  the  same  weights  and  the  same 
inertia,  and,  consequently,  are  equal  to  each  other  in  mass,  though 
the  bulk  of  the  cork  is  four  times  that  of  the  water,  and  eight  times 
that  of  the  rock.  If  the  bulks  were  equal,  the  rock  would  weigh 
twice  as  much  as  the  water,  the  cork  one  fourth  as  much,  and  the  air 
but  one  eight  hundredth  as  much.  The  specific  gravity  of  a  sub- 
stance is  such  a  comparison  of  its  weight  with  that  of  an  equal  bulk 
of  some  other  substance,  usually  water,  taken  as  a  standard.  Thus, 
the  weight  of  water  being  called  one,  the  specific  gravity  of  rock  is 
two,  of  cork  one  fourth,  of  air  one  eight  hundredth. 


-»ftr«=r.=: 


1  D 


7 

2D 


Fig.  2. 


Distance. — Although  the  attraction  of  gravitation  acts 
between  bodies,  however  far  apart  they  may  be,  the  power 
or  intensity  of  this  force  decreases  very  rapidly  as  the  dis- 
tance between  the  bodies  increases.  Thus,  gravity  acting 
from  B  is  distributed  over  four  times  the  space  at  2D,  and 
nine  times  the  space  at  ^D  that  it  is  at  \D ;  hence,  if  the 
distance  between  two  bodies  is  doubled  or  trebled,  the 
mutual  attraction  of  gravitation  which  they  exert  on  each 
other  is  decreased  to  one  fourth  and  one  ninth  respect- 
ively ;  in  other  words,  the  inte?isity  of  gravitation  varies  in- 
versely as  the  square  of  the  distance  between  the  bodies.  This 
law  applies  to  sound  and  to  radiant  heat  and  light  as  well 
as  to  gravitation. 


SOME    GENERAL    LAWS    OF    NATURE.  I  I 

The  effects  of  Gravitation  are  almost  infinite  in  num- 
ber, and  many  of  them  are  familiar  to  every  one.  In 
general,  gravitation  gives  weight  to  all  substances  in  nature — 
even  to  such  light  and  invisible  substances  as  air — and 
therefore  this  force  is  one  of  the  causes  of  all  phenomena 
in  which  the  weight  of  bodies  plays  a  part.  The  rising 
of  vapor  through  the  heavier  air,  and  the  falling  of  rain 
through  the  lighter  air ;  the  moving  of  heavy  air,  as  wind, 
to  a  region  where  the  air  is  lighter;  and  the  downward 
flow  of  streams  over  the  sloping  surface  of  the  land, — all 
these  are  caused  by  the  attraction  of  the  enormous  mass  of 
the  earth  upon  the  relatively  insignificant  masses  of  matter 
near  its  surface. 

The  effect  of  the  earth's  attraction,  however,  is  not  confined  to  the 
neighborhood  of  its  surface.  The  nearest  heavenly  body,  the 
moon,  is  much  smaller  than  the  earth  ;  it  contains  but  one  eightieth 
as  much  matter.  Though  240,000  miles  distant,  the  attraction  of  the 
larger  earth  pulls  this  body  constantly  from  the  straight  course  which 
its  inertia  influences  it  to  follow,  and  causes  the  moon's  path  or  orbit 
to  become  nearly  circular  around  the  earth.  The  moon's  attraction 
upon  the  larger  earth  is  imperceptible  on  the  solid  land,  but  it  heaps 
up  the  waters  of  the  sea  to  form  the  tides.  But  the  attraction  of 
gravitation  extends  to  all  distances.  The  sun  is  a  body  whose  mass 
is  300,000  times  as  great  as  that  of  the  earth  and  moon  together. 
The  attraction  of  this  vast  mass,  exerted  through  a  distance  of 
92,000,000  miles,  overcomes  the  earth's  inertia,  and  causes  the  earth 
and  the  other  planets  of  the  solar  system  to  move  around  the  sun  in 
nearly  circular  orbits — just  as  the  earth  influences  the  movement  of 
the  moon.  The  effect  of  the  sun's  attraction  upon  the  earth  is  seen 
in  the  regular  recurrence  of  the  seasons,  and  the  regular  variation  in 
the  length  of  the  day  and  night.  Each  fixed  star  is,  probably,  like 
our  sun,  the  center  of  a  system  of  planets.  Each  of  these  systems, 
as  a  whole,  has,  like  the  solar  system,  a  motion  of  its  own  through 
limitless  space,  which  is  modified  and  regulated  according  to  the 
mass  and  distance  of  the  various  systems  by  this  universal  force 
of  gravitation. 

Cohesion,  like  gravitation,  is  an  attractive  force  which 
may  act  on  every  molecule  in  nature.    But  it  is  unlike  gravi- 


12  PHYSICAL    GEOGRAPHY. 

tation  in  three  important  particulars:  (i)  it  acts  only  be- 
tween individual  molecules,  and  not  between  masses;  (2) 
it  acts  only  between  molecules  of  the  same  kind  of  matter ; 
and  (3)  it  acts  only  when  the  molecules  are  exceedingly 
close  together, — so  close  that  they  seem  to  be  in  absolute 
contact. 

It  is  the  force  of  gravity  acting  between  the  mass  of  the  earth  and 
the  mass  of  a  stone  which  causes  the  stone  to  fall  to  the  earth ;  but 
it  is  the  force  of  cohesion  acting  between  the  millions  of  individual 
stone  molecules  which  causes  them  to  stick  together  and  form  the 
solid  mass  of  stone.  A  man  lifting  a  bucket  from  a  well,  or  carry- 
ing a  heavy  basket,  is  thus  overcoming  the  force  of  gravity ;  while  a 
woodman  chopping  down  a  tree,  or  a  machinist  filing  a  piece  of 
iron,  is  struggling  against  the  force  of  cohesion. 

State  of  Aggregation. — It  is  believed  that  between  the 
molecules  of  all  bodies  two  antagonistic  forces  are  con- 
stantly struggling  for  mastery:  the  attractive  force  of  co- 
hesion, and  a  repulsive  force,  which  results  in  what  we  call 
"heat."  When  the  attractive  force  is  the  stronger,  the 
molecules  cohere  (stick  together)  and  form  a  solid;  when 
the  two  forces  are  about  equal,  the  molecules  move  about, 
over,  and  beside  each  other,  and  form  a  liquid;  when  the 
repulsive  force  is  the  stronger,  the  molecules  fly  farther 
apart,  and  form  a  gas. 

It  thus  depends  simply  upon  the  relative  intensities  of  cohesion 
and  heat  between  the  molecules  of  any  substance,  whether  that  sub- 
stance takes  the  form  of  a  solid,  a  liquid,  or  a  gas.  Ice,  water,  and 
steam,  for  example,  may  be  produced  from  the  same  molecules  by 
simply  making  them  colder  or  hotter.  It  is  believed  that  there  is  no 
substance  which  may  not  exist  in  any  one  of  the  three  states  of  ag- 
gregation,— a  solid,  a  liquid,  or  a  gas. 

Crystallization  is  a  peculiar  effect  of  cohesion  fre- 
quently seen  in  rock,  cast  iron,  snow,  and  very  many 
other  solids.  When  a  substance  passes  slowly  and  quietly 
from  a  liquid  to  a  solid  state,  the  molecules  generally 
arrange  themselves  in  a  peculiar  manner,  assuming  certain 


SOME    GENERAL    LAWS    OF    NATURE. 

definite  geometrical  shapes.  These  fehapes  vary  indiffer- 
ent substances,  but  remain  constant  Virt  substances  ^\he 
same  kind.  \% 

If  water,  for  instance,  be  examined  when  sofi^fylrrg^  or  /r 
delicate  needles  of  ice  will  be  observed  shooting  otTKoyer  thj 
face  and  forming  six-pointed  stars  or  little  six-sided  figures.  These 
are  ice  crystals,  and,  if  observed  in  freezing  water  in  any  part  of 
the  world,  the  ice-needles  are  always  found  to  form  angles  of  just 
6o°  with  each  other.  If  a  solution  of  table  salt  be  allowed  to  solidify, 
the  salt  takes  the  form  of  little  cubes  with  exquisitely  smooth  sides, 
and  clean,  square  angles.  These  are  salt  crystals.  In  general,  every 
substance  forms  crystals,  which  are  always  the  same  in  the  same 


12  3  4-  5 

Fig.  3. — 1.  Quartz.     2.   Gypsum.     3.   Salt.     4.   Calcspar.     5.  Feldspar. 


substance,  but  different  in  shape  from  the  crystals  of  other  sub- 
stances. Crystallization  is  explained  by  supposing  that  the  cohesive 
force  of  molecules  is  not  exerted  equally  on  all  sides,  but  is  stronger 
in  certain  definite  directions,  and  that  there  are  differences  in  this 
respect  between  the  molecules  of  different  kinds  of  matter. 

The  power  of  Cohesion  is,  of  course,  greatest  in 
solids,  but  it  varies  in  different  substances.  The  cohesive 
attraction  is  so  strong  that  it  requires  a  pull  of 

100,000  to  170,000  lbs.  to  break  a  steel      bar  i  inch  square. 
50,000  to  100,000  lbs.  "       "      "  iron         "    "     "         " 
4,000  to    20,000  lbs.  "       "      "  wooden  "    "     °         " 
500  to       1,000  lbs.  "       "      "stone      "    "     "         " 

The  change  in  the  arrangement  of  water  molecules, 
under  the  force  of  cohesion  causes  the  bursting  of  water- 
pipes,  when  the  water  in  them  crystallizes,  or  freezes. 

The  molecules  of  water  and  of  some  other  substances  occupy  a 
greater  space  when  the  absence  of  heat  allows  cohesion  to  arrange 


14  PHYSICAL    GEOGRAPHY. 

them  in  the  form  of  crystals,  than  when,  heat  being  present,  it  over- 
comes cohesion  and  arranges  the  molecules  so  that  they  slide  over 
each  other  in  the  form  of  a  liquid.  Hence,  the  contents  of  a  water- 
pipe  expands  as  it  changes  from  a  liquid  to  a  crystallized  form.  It 
expands  with  a  force  which,  at  the  ordinary  temperature  oi  freezing 
water,  produces  an  outward  pressure  of  more  than  2,000  pounds  on 
each  square  inch  of  the  interior  of  the  pipe,  and  this  pressure  in- 
creases rapidly  as  the  temperature  falls.  Few  pipes  can  withstand 
this  pressure,  and,  consequently,  they  burst.  If  the  pipe  is  strong 
enough,  however,  to  prevent  the  expansion,  the  water  can  not  freeze, 
but  remains  liquid. 

Adhesion  is  a  force  similar  to  cohesion,  except  that 
while  cohesion  acts  only  between  molecules  of  the  same 
kind  of  matter,  adnesion  acts  only  between  molecules  of 
different  kinds. 

No  body  can  become  wet  when  plunged  into  water,  if  the  adhe- 
sion between  the  molecules  of  the  water  and  the  body  is  weaker 
than  the  cohesion  between  the  molecules  of  the  water.  Grease 
is  such  a  substance,  and,  therefore,  does  not  become  wet. 

Capillary  Attraction  is  an  instance  of  adhesion,  and 
is  so  named  from  the  Latin  word  capillus,  a  hair.  When 
one  end  of  a  fine,  hair-like  tube  is  plunged  into  any  liquid 
that  will  adhere  to  the  material  of  which  the  tube  is  made, 
the  attraction  of  adhesion  causes  the  liquid  to  rise  in  the 
tube  a  short  distance  above  the  level  of  the  liquid  outside 
the  tube.  The  finer  the  bore  of  the  tube,  the  higher  will 
the  liquid  rise  in  it. 

If  the  corner  of  a  towel  be  allowed  to  remain  for  a  short  time  in 
water,  the  towel,  for  some  distance  above  the  surface  of  the  water 
will  be  found  to  be  wet,  the  minute  spaces  between  the  strands  of 
the  threads  having  acted  as  so  many  tubes  in  which  the  water  rises 
by  capillary  attraction. 

Capillary  attraction  plays  a  very  important  part  in  nature ;  it 
enables  the  soil  and  rocks  to  retain  water,  it  forms  one  of  the 
means  by  which  plants  are  supplied  with  their  liquid  food,  and  it  is 
called  into  use  in  distributing  some  of  the  animal  juices  through- 
out the  body. 


SOME    GENERAL    LAWS    OF  NATURE.  1 5 

Chemical  Affinity,  like  gravitation,  is  an  attractive 
force,  and,  like  cohesion,  acts  only  at  imperceptible  dis- 
tances. But  it  is  an  atomic  force ;  it  acts,  primarily,  only 
between  atoms,  and  when  these  atoms  are  of  different 
kinds  of  matter,  a  substance  is  produced  entirely  different 
from  either  of  the  atoms. 

The  difference  between  affinity  and  cohesion  is  thus  very  appar- 
ent ;  cohesion  increases  the  mass  of  a  substance  by  adding  together 
many  minute  particles  of  the  same  substance ;  affinity  produces  a 
new  substance  by  combining  still  more  minute  particles  of  totally 
dissimilar  substances. 

Elements. — It  has  been  said  that  an  element  or  simple 
substance  is  composed  of  but  a  single  kind  of  matter; 
that  is,  if  an  element  could  be  broken  up  into  its  atoms, 
all  the  atoms  would  be  exactly  alike  in  all  particu- 
lars. There  are  about  seventy  known  elements ;  of  these, 
fifty-five  are  called  metals,  such  as  aluminium,  calcium, 
potassium,  iron,  copper,  gold,  etc.  The  remaining  ele- 
ments are  called  metalloids ;  some  are  solids  at  ordinary 
temperatures,  such  as  silicon,  carbon  (charcoal),  phosphorus, 
and  sulphur;  one  is  a  liquid,  —  bromine;  and  others  are 
gases  at  ordinary  temperatures,  such  as  oxygen,  hydrogen, 
and  nitrogen. 

Compounds. — Every  substance  in  nature  is  composed 
of  one  or  more  elements  in  a  free  state,  or  is  a  chemical 
combination  of  two  or  more  elements.  Such  a  combina- 
tion produces  a  compound  substance. 

Air  is  composed  chiefly  of  oxygen  and  nitrogen  mixed  together 
in  the  free  states.  Water  is  a  chemical  compound  of  the  two  ele- 
ments, oxygen  and  hydrogen  ;  quartz ,or  fiint,  of  oxygen  and  silicon; 
limestone,  of  oxygen,  carbon,  and  calcium ;  bread,  meat,  and  most 
foods,  of  oxygen,  hydrogen,  nitrogen,  and  carbon,  together  with 
minute  quantities  of  various  other  elements. 

Oxygen  is  far  the  most  abundant  element  on  the  earth. 
At  ordinary  temperatures  it  is  a  gas — colorless,  tasteless, 

P.  G—«. 


1 6  PHYSICAL    GEOGRAPHY. 

and  odorless,    and  a  trifle  heavier  than  air.     Oxygen  is 
essential  to  all  life,   both  animal  and  vegetable. 

Oxygen  forms  about  \  by  weight  of  all  rocks  of  the  earth. 

.<  8      «  M  «       ««    water> 

it  „        £    ..  ,<         u      „    air> 

"      f   "        "       "     *'  vegetable  matter. 
"  "       §  "         "       "     "  animal  matter. 

Oxidation  and  Combustion. — The  affinity  of  oxygen 
for  most  of  the  substances  in  nature  is  so  strong  that  it  is 
constantly  combining  with  them  when  they  are  exposed 
to  the  air  with  its  large  quantity  of  free  oxygen.  The 
process  is  called  li  rusting,"  "rotting,"  or  "burning"  in 
different  instances,  all  of  which  are  covered  by  the  general 
term  oxidation.  Oxidation  always  produces  heat,  as  is 
the  case  with  most  chemical  combinations.  When  the 
heat  is  great  enough  to  produce  light,  the  process  is 
called  combustion. 

A  familiar  instance  of  oxidation  is  the  "  rusting  "  of  iron  in  the 
air  The  oxygen  of  the  air  unites  with  the  surface  iron  to  form  the 
new  substance,  oxide  of  iron.  The  burning  of  fuel  is  really  the 
same  process,  only  the  chemical  affinity  between  the  carbon  and 
hydrogen  of  which  most  fuel  is  composed,  and  the  oxygen  of  the 
air,  is  much  greater  than  that  between  iron  and  oxygen ;  the  heat 
produced  by  their  union  is  great  enough  to  produce  light,  and  the 
process  is  consequently  called  combustion. 

Decomposition  is  the  breaking  up  of  a  compound  into 
its  elements  or  into  simpler  compounds.  The  affinity  be- 
tween different  elements  varies  in  strength.  Whenever 
elements  having  a  stronger  affinity  for  each  other  than 
for  those  with  which  they  happen  to  be  combined,  are 
brought  by  any  means  sufficiently  close  together  for  affinity 
to  act,  the  weak  combinations  break  up,  and  the  ele- 
ments, liberated  by  the  decomposition,  recombine  accord- 
ing to  the  strength  of  their  mutual  affinities  to  form  other 
substances,  which,  under  similar  circumstances,  decompose 


SOME   GENERAL    LAWS    OF    NATURE.  IJ 

and  recombine  into  still  other  compounds.     In  this  way, 
the  atoms  and  molecules  are  kept  in  circulation. 

Gunpowder  is  nothing  but  certain  weak  combinations  containing 
elements  that  have  a  strong  affinity  for  each  other,  artificially  placed 
so  close  together  that  a  slight  heat  causes  the  weak  combinations  to 
decompose  and  leave  certain  of  their  ingredients  free  to  unite  in  a 
strong  combination. 

Kinetic  Energy. — One  or  more  of  these  three  great 
attractive  forces  —  gravitation,  cohesion,  or  affinity  —  is 
thought  to  be,  directly  or  indirectly,  the  cause  of  all  mo- 
tion in  the  universe;  and  every  one  of  the  innumerable 
changes  that  are  constantly  taking  place  in  matter  around 
us,  is  thought  to  be  the  result  of  some  kind  of  motion  of 
that  matter, — either  a  visible  motion  of  its  mass  as  a 
whole,  or  an  invisible  motion  of  its  molecules  or  atoms. 
Matter  in  motion  always  imparts  some  kind  of  motion  to 
other  matter  with  which  it  comes  in  contact.  This  im- 
parting of  motion  is  called  doing  work.  Matter  in  motion 
is,  therefore,  said  to  have  the  power  to  do  work,  or  to 
possess  kinetic  (active)  energy. 

The  quantity  of  work  a  body  in  motion  is  capable  of  doing, —  or 
the  amount  of  kinetic  energy  it  possesses, — depends  more  upon  the 
rapidity  of  its  motion  than  upon  its  mass ;  for,  while  doubling  the 
mass  only  doubles  the  energy,  doubling  the  speed  increases  the 
energy  four  times ;  thus,  energy  increases  with  the  mass  of  a  body 
but  with  the  square  of  its  velocity.  That  is,  a  hundred-pound  cannon- 
ball,  moving  with  a  certain  speed,  possesses  no  more  energy  than  a 
one-pound  ball  moving  ten  times  as  fast. 

Potential  Energy. — It  often  happens  that  the  expendi- 
ture of  kinetic  energy  upon  a  body  places  it  in  such  a 
position  that,  though  not  in  motion,  it  would  move  and 
do  work  if  unrestrained.  A  body  in  such  a  position  is 
said  to  possess  potential  {possible)  energy.  A  bent  bow,  a 
hoisted  weight,  or  a  wound-up  watch  spring  possesses  po- 
tential energy. 


1 8  PHYSICAL    GEOGRAPHY. 

Potential  energy  is  thus  simply  stored-up  kinetic  energy,  since  it 
exists  only  in  bodies  on  which  kinetic  energy  has  been  expended. 
Upon  the  removal  of  restraint,  the  body  immediately  moves,  and  the 
potential  energy  is  converted  back  again  into  exactly  the  amount  of 
kinetic  energy  which  was  expended  in  its  production. 

Conservation  of  Energy  means  that  the  total  amount 
of  energy  in  the  universe  is  an  unchangeable  quantity. 
Energy,  therefore,  can  not  be  created  or  destroyed,  but  it 
is  constantly  eluding  observation  by  changing  its  form,  or 
by  entering  and  leaving  different  masses  of  matter.  It  is 
frequently  apparently  destroyed  either  (i)  when  it  changes 
from  a  kinetic  to  a  potential  form,  —  as  when  the  kinetic 
energy  expended  in  lifting  a  heavy  weight  is  changed  into 
potential  energy  of  the  weight  when  the  latter  stops  moving 
just  before  falling ;  or  (2)  when  masses  in  visible  motion  impart 
a  portion  or  all  of  their  motion  and  energy  to  invisible 
molecules,  —  as  when  a  falling  weight  strikes  the  earth,  and 
its  motion,  as  a  mass,  stops.  In  this  case,  if  the  weight 
and  the  place  struck  were  carefully  examined,  they  would 
be  found  to  have  undergone  certain  changes  in  conse- 
quence of  the  blow ;  they  would  be  hotter  than  before, 
and  they  might  have  become  luminous,  or  other  changes 
might  have  taken  place. 

These  changes — heat,  light,  etc., — are  simply  the  results  of  molec- 
ular energy  arising  from  the  invisible  motions  imparted  to  the  mole- 
cules by  the  collision  which  stopped  the  visible  motion  of  the  weight. 
If  the  energy  causing  all  the  changes  that  occurred  in  consequence 
of  the  blow  could  be  collected,  it  would  be  found  to  equal  the  amount 
which  the  weight  possessed  when  it  struck  the  earth,  and  exactly  this 
amount  of  energy  is  passed  on  to  other  matter  by  the  molecules  be 
fore  they  regain  their  former  condition. 

Nature  of  Heat  and  Light. — Heat  and  light  are  the 
results  of  a  certain  kind  of  insensible  motion  of  the  mole- 
cules of  matter.  Therefore,  all  warm  or  luminous  bodies 
possess  kinetic  energy  by  virtue  of  this  motion.     A  body 


SOME  GENERAL  LAWS  OF  NATURE.        1 9 

is  said  to  be  hot  when  its  molecules  possess  an  exceed- 
ingly rapid,  but  of  course  invisible,  vibratory  motion. 
When  the  molecular  motions  increase  to  a  certain  rapidity, 
the  body  becomes  luminous,  and  is  said  to  be  "red"  hot. 
As  the  motions  become  slower,  the  body  ceases  to  be 
luminous  and  becomes  cooler,  but  the  molecules  of  no  body 
are  supposed  to  be  at  rest.  Hence  all  bodies,  even  the 
coldest,  are  thought  to  have  more  or  less  heat.  Whatever 
increases  the  rapidity  of  the  motions  increases  the  heat  of 
the  body,  and  whatever  decreases  the  rapidity  causes  the 
body  to  cool. 

Repeated  blows  of  a  hammer  on  a  nail,  or  the  friction  of  one 
body  rubbing  on  another,  increases  the  rapidity  of  the  molecular 
agitation  in  each,  and  thus  produces  heat  mechanically.  The  clash 
of  atoms  colliding  under  the  attraction  of  affinity,  produces  a  sim- 
ilar increase,  and  produces  heat  chemically.  The  heat  of  all  "fire" 
is  thus  produced. 

Transference  of  Heat. — Unequally  heated  bodies, 
whether  touching  each  other  or  not,  always  tend  to  acquire 
a  uniform  temperature,  the  hotter  becoming  cooler,  and 
the  cooler  becoming  hotter.  This  is  accomplished  (i)  by 
radiation  of  energy  ;  (2)  by  conduction ;  or  (3)  by  con- 
vection. 

Radiation  of  Energy  takes  place  between  unequally 
heated  bodies  that  are  not  in  contact.  It  is  explained  by 
supposing  that  the  universe  is  pervaded  by  an  elastic  sub- 
stance called  luminiferous  ether,  so  thin  that  it  enters  and  com- 
pletely fills  the  invisible  interstices  between  the  molecules 
of  all  substances  as  easily  as  water  fills  the  cavities  of  a 
sponge.  The  movements  of  the  molecules  of  all  bodies 
tend  to  produce  vibrations  in  the  ether  pervading  and 
surrounding  them,  just  as  any  shock  sets  a  bowl  of  jelly 
in  a  quiver.  The  hotter  bodies  tend  to  impress  quicker 
vibrations  on  the  ether  than  the  cooler  ones.     A  continued 


20  PHYSICAL    GEOGRAPHY. 

expenditure  of  energy  (heat)  is  required  to  maintain  the 
vibrations  of  the  ether,  which  spread  away  or  radiate  in  all 
directions ;  hence  the  body  that  excites  the  vibrations  cools ; 
but  if  the  energy  of  the  vibrations  of  the  ether  is  expended 
upon,  or  absorbed  by,  a  substance  whose  molecules  are 
thereby  excited  to  faster  motion,  the  substance  is  warmed. 
Radiant  energy  is  said  to  pass  from  one  body  to  another  in 
rays.  Some  of  the  rays  emitted  by  very  hot  bodies  are 
perceptible  to  the  nerves  of  the  eye  as  light;  these  rays 
have  392  trillion  to  757  trillion  vibrations  a  second.  Rays 
of  slower  or  faster  vibrations  are  not  perceived  by  the  eye. 
Any  ray  that  is  absorbed  by  a  substance,  whether  visible 
or  invisible  as  light,   produces  heat. 

It  is  by  radiation  of  energy  that  the  heat  and  light  of  the  sun 
reach  the  earth,  and  that  a  person  is  warmed  when  standing  before 
a  fire.  Radiant  energy  travels  through  ether  at  the  enormous  speed 
of  186,000  miles  a  second.  The  heat  and  light  of  the  sun  require 
about  eight  minutes  to  readi  the  earth. 

Conduction. — When  unequally  heated  bodies  or  mol- 
ecules are  so  close  together  that  they  are  usually  said  to  be 
in  contact — as  the  molecules  are  in  solids — the  more  active 
molecules  impart  some  of  their  motion  to  the  slower  mov- 
ing adjacent  molecules,  and  these  to  their  still  slower 
neighbors,  until  a  uniform  heat  and  rate  of  motion  is  con- 
ducted to  the  most  distant  part  of  the  body.  In  compar- 
ison with  radiation,  the  conduction  of  heat  is  exceedingly 
slow;  but  dense  bodies,  such  as  the  metals,  conduct  heat 
faster  than  porous  bodies,  such  as  snow,  earth,  rock,  etc. 
The  former  are  therefore  called  good  conductors,  while  the 
latter  are  called  poor  or  non-conductors. 

Convection.— Liquids  and  gases  are  very  poor  conduc- 
tors, since  their  molecules  can  move  freely  among  them- 
selves. Hence,  if  the  upper  part  of  a  liquid  or  gas  is 
warmed,  a  very  long  time  is  required  to  transfer  heat  to 


SOME    GENERAL    LAWS    OF   NATURE.  21 

the  lower  portion;  but  if  heat  is  applied  from  below,  the 
lower  portions  generally  expand  as  they  grow  warmer,  and 
thus  become  lighter  than  those  above.  The  lower  portions 
are  therefore  forced  to  rise  by  the  gravity  of  the  heavier 
portions  above,  and  thus  convection  currents  are  established, 
which  convey  the  heat  throughout  the  liquid  or  the  gas. 
Reflection,  Absorption,  and  Transmission  of  Ra- 
diant Energy. — When  radiant  energy  encounters  a  body, 
it  (i)  enters  the  body,  or  (2)  is  reflected  from  its  sur- 
face. That  which  enters  may  be  either  transmitted  through 
the  body  and  pass  out  on  the  opposite  side,  or  it  may  be 
absorbed  (retained  in  the  body).  It  is  only  the  energy 
that  is  absorbed  that  affects  the  temperature  of  the  body. 
Bodies  are  called  good  reflectors,  absorbers,  or  transmit- 
ters of  radiant  energy  according  as  they  reflect,  absorb,  or 
transmit  the  greater  part  of  the  rays  falling  on  their  sur- 
face, though  no  body  is  perfect  in  either  respect ;  the  best 
reflectors  absorb  some  of  the  energy,  the  best  absorbers 
reflect  a  portion,  and  the  best  transmitters  both  absorb 
and  reflect  a  little. 

Bodies  such  as  glass,  which  transmit  most  of  the  rapid  vibra- 
tions of  visible  rays,  are  called  transparent.  Bodies  which  transmit 
most  of  the  ether  vibrations  of  either  visible  or  invisible  rays,  with- 
out being  themselves  warmed,  are  called  diathermanous.  Bodies 
which  absorb  most  of  the  ether  vibrations,  and  are  hence  warmed, 
are  said  to  be  athermanous.  Most  bodies  are  athermanous,  and  none 
are  perfectly  diathermanous  or  transparent.  Dry  air  and  rock  salt 
are  among  the  most  diathermanous  substances.  The  dry,  pure  air  of 
high  mountains  transmits  nearly  all  the  heat  of  the  sun's  rays,  and  is 
itself  scarcely  warmed  by  them.  On  account  of  this  quality  of  the 
air,  a  person  at  the  top  of  a  very  high  mountain  might  be  quite  un- 
comfortably warm  in  the  sunshine,  and  yet  water  might  be  freezing 
in  the  shadow  of  a  rock  beside  him.  Glass,  and  air  rendered  slightly 
hazy  by  fine  water  globules  or  dust  particles,  though  diathermanous 
to  light  rays,  are  largely  athermanous  to  rays  emitted  by  dark  bodies. 
Thus,  the  window-glass  allows  the  sunbeams  to  enter  and  warm  a 
room,  but  prevents  the  dark  radiations  from  the  warm  interior  from 


22  PHYSICAL  GEOGRAPHY. 

passing  out  again.    Water,  though  exceedingly  transparent,  transmits 
scarcely  any  dark  rays. 

Expansion  and  Contraction. — When  a  body  grows 
hotter  or  colder,  a  change  in  its  size  always  takes  place. 
As  a  general  rule,  bodies  expand  when  heated  and  contract 
when  cooled. 

In  the  ordinary  thermometer,  or  heat  measure,  the  expansion  and 
contraction  is  employed  to  denote  its  change  of  temperature.  The 
common  thermometer  consists  of  a  glass  tube  of  very  fine 
bore,  terminating  in  a  bulb,  which,  with  part  of  the  tube,  is 
filled  with  some  liquid,  usually  mercury.  As  the  tempera- 
ture of  the  mercury  increases,  it  expands  and  mounts  higher 
in  the  tube ;  as  the  temperature  decreases,  the  mercury 
contracts  and  descends  in  the  tube.  The  tube  is  graduated 
by  marking  the  height  of  the  mercury  when  the  bulb  is 
held  first  in  freezing  and  then  in  boiling  water,  and  marking 
the  intervening  space  into  equal  divisions  called  degrees. 
In  Fahrenheit's  thermometer,  which  is  generally  used  in 
this  country,  the  freezing  point  is  marked  320  and  the  boil- 
ing point  2120.  When  the  thermometer  is  brought  into  the 
neighborhood  of  a  hot  or  cold  body,  the  mutual  radiation 
between  the  body  and  the  instrument  reduces  them  to  a 
common  temperature,  which  can  at  once  be  compared  with 


Fie-  4-  that  of  freezing  or  boiling  water  by  noting  the  height  of  the 
mercury  in  the  graduated  tube. 

The  power  with  which  substances  expand  or  con- 
tract is  practically  resistless.  The  amount  of  expansion  or 
contraction  varies  in  different  substances ;  thus,  for  each 
degree  of  variation  in  temperature,  a  mass  of  air  grows 
larger  or  smaller  by  about  -nftftftftnr;  water,  T^HKhnr  J  ice> 

nnWtar;  iron>  -nnr2oW;  and  rock  but  tttoVW  of  its 
bulk   or  volume.     These  amounts  are  so  small  as  to  be 

usually  imperceptible,  but  when  substances  are  in  large 
quantity,  or  when  the  variation  in  temperature  is  great, 
the  expansion  or  contraction  is  very  perceptible ;  and,  be- 
ing resistless,  the  results  are  stupendous. 


SOME    GENERAL    LAWS    OF  NATURE. 

A  change  of  temperature  of  io°  in  a  mass  of  air  one  rmle  square 
tends  to  change  its  length,  breadth,  and  thickness  by  about  thirty-six 
feet.  A  much  greater  change  of  temperature  occurs  daily  through 
millions  of  cubic  miles  of  the  atmosphere.  A  change  of  only  f°  in r 
the  temperature  of  a  sheet  of  ice  a  mile  long  changes  its  length 
about  1%  inches.  A  contraction  of  even  this  small  amount  accounts 
for  the  long,  fine  cracks  which  open  with  loud  report  in  the  ice  of  all 
ponds  and  lakes  in  cold  weather.  A  part  of  the  earth's  rocky  crust 
one  mile  in  length  would  tend  to  become  about  2}i  feet  longer  were 
its  temperature  increased  ioo°. 

Explanation  of  Expansion   and  Contraction. — The 

expansion  or  contraction  of  a  body  results  from  a  move- 
ment of  its  molecules  into  an  arrangement  occupying  a 
greater  or  a  less  space.  Cohesion  usually  resists  such 
movements  as  result  in  expansion;  hence,  part  of  the 
heat-energy  entering  a  body  must  counteract  the  resistance 
of  cohesion,  and  is  thus  held  in  a  potential  or  inactive  form 
to  maintain  the  altered  size  of  the  body,  and  only  the  re- 
mainder of  the  energy  is  left  to  cause  the  change  in  the 
active  motion  or  quiver  of  the  molecules,  which  alters  the 
temperature.  Conversely,  when  a  body  cools  and  contracts, 
it  surrenders  not  only  a  portion  of  its  active,  temperature- 
maintaining  energy,  but  also  a  portion  of  its  potential, 
size-maintaining  energy,  which,  being  relieved  of  the  re- 
sistance of  cohesion,  becomes  active  heat-energy  as  it 
leaves  the  body.  The  resistance  of  cohesion  is  very 
different  in  different  substances ;  hence,  the  amount  of 
heat-energy  required  to  produce  the  same  change  of  tem- 
perature varies  greatly  in  different  substances. 

Water  requires  a  greater  amount  than  almost  any  other  substance. 
Thus,  if  all  the  energy  liberated  in  cooling  a  mass  of  rock  50  were 
to  enter  an  equal  weight  of  water,  it  would  raise  its  temperature  but 
i°.  Conversely,  when  a  given  weight  of  water  cools  i°,  it  liberates 
enough  energy  to  raise  the  temperature  of  an  equal  weight  of  rock  50. 
Water  is  therefore  said  to  have  a  capacity  for  heat,  or  a  specific 
heat,  five  times  as  great  as  rock.  The  specific  heat  of  water  is  about 
four  times  that  of  air. 


24  PHYSICAL    GEOGRAPHY. 

On  account  of  its  great  specific  heat,  water  cools  or  is  heated 
more  slowly  than  almost  any  other  substance.  Thus,  if  a  pound  of 
water,  at  500  temperature,  is  surrounded  by  a  pound  of  air  at  450 
temperature,  the  water  cools  and  the  air  becomes  warmer  until  their 
temperatures  are  the  same  ;  but  the  water  cools  only  i°,  for  in  doing 
so  it  liberates  enough  energy  to  heat  the  air  40 ;  hence,  the  resulting 
uniform  temperature  of  air  and  water  is  490.  The  bulk  of  a  pound 
of  air  is  about  840  times  larger  than  that  of  a  pound  of  water; 
hence,  a  pond  a  foot  deep,  in  cooling  i°,  liberates  enough  energy  to 
heat  by  40  the  overlying  air  to  a  height  of  840  feet;  thus,  large 
bodies  of  water  have  a  powerful  influence  upon  the  climate  in  their 
neighborhood. 

Latent  Heat. —  All  bodies  require  an  exceptionally 
large  amount  of  energy  to  effect  the  peculiar  re-arrange- 
ment of  their  molecules  when  they  change  from  a  solid  to 
a  liquid,  or  from  a  liquid  into  a  gaseous  state.  When  a 
solid  is  heated,  its  size  and  temperature  increase  until  it 
begins  to  melt;  then,  though  heat  is  still  applied,  its  tem- 
perature remains  unchanged  until  all  of  it  is  melted,  the 
entire  energy  of  the  heat  being  required  to  re-arrange  the 
molecules  into  a  liquid  form.  When  this  re-arrangement 
of  all  the  molecules  is  completed,  if  heat  be  still  applied, 
the  size  and  temperature  of  the  liquid  increase  until  it 
begins  to  boil  or  pass  into  vapor.  Here  the  same  thing 
happens ;  although  heat  is  applied  continuously,  all  its 
energy  is  rendered  potential  by  the  resistance  which  cohe- 
sion offers  to  the  alteration  of  molecular  arrangement  into 
a  gaseous  form,  and  the  temperature  remains  unchanged 
until  the  liquid  has  entirely  disappeared,  after  which  the 
temperature  of  the  gas  begins  to  increase. 

The  energy  which  thus  disappears  upon  the  melting  or 
vaporizing  of  substances,  is  said  to  become  latent  (con- 
cealed) ;  for  when  the  substance  passes  back  again  into  a 
solid  or  liquid  state  upon  cooling,  the  latent  energy  again 
appears  as  heat,  which  affects  the  temperature  of  sur- 
rounding bodies. 


SOME    GENERAL    LAWS   OF  NATURE.  25 

The  latent  heat  of  water  is  greater  than  that  of  most  other  sub- 
stances. It  requires  as  much  heat-energy  simply  to  melt  a  pound  of 
ice — without  changing  its  temperature  in  the  least — as  is  required  to 
raise  the  temperature  of  140  pounds  of  water  i°,  while  the  energy 
required  to  vaporize  a  pound  of  water  would  raise  the  temperature 
of  980  pounds  of  water  i°. 

Freezing  of  Water. — Water,  iron,  and  some  other 
substances  occupy  a  greater  space  in  the  solid  than  in 
the  liquid  state,  and  hence  do  not  expand  and  contract 
according  to  the  general  rule  when  near  their  melting  points. 
If  fresh  water  be  cooled,  it  contracts  regularly  till  it 
reaches  its  maximum  density  at  a  temperature  of  390,  after 
which  it  slowly  expands  as  it  cools,  until,  in  freezing,  it 
makes  a  sudden  and  great  expansion — twelve  cubic  inches 
of  water  making  about  thirteen  cubic  inches  of  ice.  Ice 
is  consequently  lighter  than  an  equal  bulk  of  water,  and 
hence  floats.  If  the  ice  be  further  cooled,  it  will  be 
found  to  contract  regularly.  Hence,  it  is  only  during 
the  change  from  the  liquid  into  the  solid  state  that  the 
general  rule  of  expansion  and  contraction  is  reversed. 

This  property  of  water  is  of  great  importance.  Lakes  and  rivers 
cool  in  winter  by  radiating  and  conducting  heat  to  the  colder 
air.  As  the  surface  water  cools  and  contracts,  it  sinks,  and  is  re- 
placed by  warmer  water  from  beneath,  which  in  turn  cools  and 
sinks,  until  the  whole  depth  of  the  water  is  reduced  to  390.  Should 
this  process  continue  until  ice  was  formed,  the  ice,  too,  would  sink, 
and  accumulate  at  the  bottom  until  the  lakes  and  streams  were 
converted  into  solid  blocks  of  ice,  which  the  heat  of  the  succeed- 
ing summer  could  not  melt.  But,  after  reaching  390,  the  water 
expands  by  cooling  until  after  ice  is  formed.  Hence,  the  ice-cold 
surface  water  and  the  ice  are  lighter  than  the  deeper  water,  and 
form  a  floating  blanket,  which  prevents  to  a  great  extent  the  escape 
of  heat  from  the  slightly  warmer  water  beneath,  and  so  preserves  it 
in  a  liquid  state  through  the  winter. 

Evaporation  is  the  process  by  which  many  liquids  and 
some  solids  pass  into  a  gaseous  state  at  temperatures  far 
below   their  boiling  points.     Evaporation   is  almost   con- 


26  PHYSICAL    GEOGRAPHY. 

stantly  taking  place  at  the  surface  of  every  sheet  of  water, 
snow,  or  ice,  as  well  as  at  every  moist  surface  in  the  world. 
It  is  made  strikingly  manifest  when  a  damp  cloth  is  hung 
in  the  air,  for  in  a  short  time  the  cloth  becomes  dry — 
that  is,  the  moisture  evaporates  and  passes  off  into  the  air 
in  the  form  of  invisible  water-gas,   or  vapor. 

Although  evaporation  takes  place  at  temperatures  much  lower 
than  the  boiling  point  of  water,  the  amount  of  energy  rendered 
"latent"  is  about  the  same  in  both  processes.  Energy  in  the  case 
of  evaporation  is  supplied  by  the  molecular  motion  of  surrounding 
substances,  which  thus  become  cooler.  This  accounts  for  the  sen- 
sation of  cold  when  a  rapidly  evaporating  liquid,  as  cologne  or 
ammonia,  is  poured  on  the  hand ;  part  of  the  energy  employed  in 
maintaining  the  temperature  of  the  hand  is  drawn  upon  to  re- 
arrange the  molecules  of  the  liquid  and  maintain  it  in  a  gaseous 
state ;  this  energy  thus  disappears  as  heat,  or  becomes  latent. 

Mechanical  Equivalent  of  Heat. — Exactly  the  same 
amount  of  energy  is  always  required  under  similar  condi- 
tions to  increase  the  heat  of  a  substance  from  one  given 
temperature  to  another;  and  conversely,  in  cooling  from 
a  given  temperature  to  another,  a  body  always  liberates 
exactly  the  same  amount  of  energy.  The  amount  of 
energy  required  to  raise  the  temperature  of  a  pound  of 
water  i°,  and  to  effect  its  corresponding  expansion,  is 
equal  to  that  possessed  by  a  one-pound  weight  striking 
the  earth  after  a  fall  of  772  feet.  This  amount  of  energy 
is  called  the  mechanical  equivalent  of  heat. 

The  enormous  energy  of  heat  is  thus  made  manifest :  a  cubic  foot 
of  water  (62^  pounds),  can  never  be  heated  from  the  freezing  to  the 
boiling  point  except  by  the  expenditure  of  enough  energy  to  raise 
bodily  a  large  locomotive  engine  and  tender  (43 %  tons)  100  feet 
high  into  the  air;  and  whenever  a  cubic  foot  of  water  simply  cools 
from  the  boiling  to  the  freezing  point,  enough  energy  is  liberated 
to  accomplish  this  same  enormous  lift. 

Refraction. — A  ray  of  light  passes  through  a  trans- 
parent body  in  a  straight  line;    but   in  passing  obliquely 


SOME    GENERAL    LAWS   OF    NATURE. 


27 


Fig.  5- 


from  any  transparent  body  to  another  of  different  density, 
as  from  air  to  water,  water  to  air,  or  air  to  glass,  the  path 
of  the  ray  is  bent  from  a  straight  course.  This  bending  is 
called  refraction. 

Thus,  suppose  the  ray  ab  (Fig.  5)  to  be  passing  obliquely  from  the 
air  into  the  denser  transparent  substance,  glass. 
At  b  part  of  the  ray  is  reflected  toward  g; 
part  is  absorbed  by  the  glass,  and  the  rest  is 
refracted  in  the  direction  be.  At  c  part  of  this 
is  reflected  back  toward  h,  and  the  rest,  upon 
re-entering  the  air,  is  again  refracted  in  the 
direction  cd.  If  the  surfaces  of  the  glass  are 
not  parallel,  but  form  the  sides  of  a  triangular  prism,  as  in  Fig.  6, 
then  the  incident  ray  ab,  from  the  candle,  will  be  refracted  in  the 

direction  cd.  As  objects  appear  in  the 
direction  from  which  the  ray  enters  the 
eye,  the  candle  a  would  appear  to  an 
observer  at  d  to  be  at  x.  If  the  inci- 
fcT*  dent  ray  fall  very  obliquely  on  the  re- 

q^  .  fracting   surface,  it  is  totally  reflected 

from  that  surface,  and  does  not  pene- 
trate it  at  all.  Thus,  when  a  ray  from  an  object  at  the  bottom  of  a 
pond  makes  an  angle  greater  than  480  27'  with  a  perpendicular  to 
the  surface  of  the  water,  it  does  not  enter  the  air,  but  is  totally  re- 
flected from  the  surface  toward  the  bottom  again. 

Diffusion  of  Light. — If  a  ray  of  sunlight  enters  a 
completely  darkened  room  through  a  small  aperture  and 
falls  upon  a  screen,  (1)  the  path  of  the  ray  becomes  visi- 
ble from  the  illumination  of  the  floating  dust  and  air 
particles;  (2)  a  bright  white  image  of  the  aperture  is 
formed  on  the  screen;  and  (3)  the  light,  reflected  from 
the  dust  and  air  particles,  and  from  the  image  on  the 
screen,  is  diffused  throughout  the  room,  and  the  outlines 
of  objects  in  it  become  visible. 

Were  it  not  for  this  general  reflection  and  diffusion  of  light  by 
the  particles  of  the  atmosphere,  all  shadows  would  be  perfectly 
black,  and  all  objects  in  shadow  would  be  invisible. 


Fig.  7- 


28  PHYSICAL    GEOGRAPHY. 

The  Spectrum. — If  a  triangular  prism  of  glass  be  held 
in  the  ray  between  the  aperture  and  the  screen,  the- bright 

white  image  not  only  alters 
its  position  by  the  refraction 
of  the  ray,  but  it  changes 
into  an  elongated,  variously 
colored  band  (Fig.  7),  the 
colors  shading  off  impercept- 
ibly from  red  at  one  end,  through  orange,  yellow,  and 
green,  to  a  pale  blue  or  violet  at  the  other  end.  This 
colored  band  of  light  is  called  the  solar  spectrum. 

The  spectrum  is  explained  by  supposing  that  the  sensation  of 
color  depends  entirely  upon  the  rapidity  of  the  ether  vibrations  or 
waves,  which  produce  light.  When  the  rate  of  vibration  is  392 
trillions  to  the  second,  the  sensation  of  red  is  produced  upon  the 
eye.  As  the  vibrations  increase  in  rapidity,  they  give  rise  succes- 
sively to  each  of  the  color  sensations  of  the  spectrum.  If  the 
rapidity  of  vibration  increases  beyond  that  which  produces  the  sen- 
sation of  violet  (757  trillions  to  the  second),  the  eye  is  not  affected, 
and  they  cease  to  be  luminous.  A  ray  of  sunlight  is  composed  of 
vibrations  of  all  degrees  of  rapidity  which  collectively  produce  a 
white  or  colorless  sensation.  By  refraction,  the  ether  waves  of 
different  degrees  of  rapidity  are  separated,  the  more  rapid  waves  be- 
ing bent,  or  refracted,  more  than  the  slower  ones ;  thus,  the  elon- 
gated band  or  image  is  produced,  each  part  of  which  reflects  to  the 
eye  waves  of  a  different  rapidity,  and  hence  produces  different  color 
sensations.  The  color  of  any  object  in  nature  depends  upon  the 
rapidity  of  the  ether  waves  which  it  is  able  to  reflect  or  transmit  to 
the  eye ;  thus,  red  glass  absorbs  the  energy  of  all  the  luminous 
ether  waves  except  that  of  the  slowest,  which  it  is  able  to  transmit ; 
these  rays  produce  the  red  sensation,  and  the  glass  appears  of  that 
color.  A  leaf,  in  the  same  way,  absorbs  all  ether  waves  excepting 
those  which  on  reflection  excite  the  green  sensation.  This  absorp- 
tion by  different  bodies  of  ether  waves  of  certain  length,  and  the 
transmission  or  reflection  of  those  of  other  length,  is  called  selective 
absorption. 

Magnetism  and  Electricity  are  peculiar  states  or  con- 
ditions of  matter,  probably  of  the  luminiferous  ether,  pro- 


SOME   GENERAL    LAWS    OF    NATURE.  29 

duced  by  the  expenditure  of  kinetic  energy  upon  it.  The 
exact  nature  of  these  conditions  is  very  imperfectly  under- 
stood, but  many  of  the  peculiarities  which  they  induce  in 
ordinary  matter  have  long  been  recognized. 

Under  the  influence  of  the  magnetic  condition,  a 
body  exercises  an  attractive  or  a  repellent  force  upon  other 
matter  in  its  neighborhood,  and  is  called  a  magnet.  The 
neighborhood  over  which  it  exerts  this  force  is  called  its 
magnetic  field. 

A  kind  of  iron  ore  (lodestone)  is  always  magnetic,  and  attracts 
certain  substances.  Small  pieces  of  iron,  for  instance,  will  move  to 
the  lodestone  over  short  distances,  and  adhere  to  it,  and  while  under 
its  influence  are  themselves  magnetic.  Soft  iron,  however,  loses 
this  quality  upon  the  removal  of  the  lodestone,  and  is,  therefore, 
called  a  temporary  magnet.  Hard  iron  and  steel,  on  the  contrary, 
retain  the  magnetic  properties  of  the  lodestone,  and  become  perma- 
nent magnets.  Magnetism  is  said  to  have  been  imparted  to  these 
bodies  by  induction. 

Poles  of  a  Magnet. — The  attraction  of  a  magnet  is 
not  uniform  throughout  its  length,  but  is  greatest  near  its 
ends,  which  are  called  poles.  Thus,  if  a  magnet  be  laid 
among  iron  filings,  they  will  adhere  in  great  tufts  to  the 
ends  or  poles,  but  not  to  the  center  of  the  magnet. 
Whenever  a  body  is  magnetized,  however  small  it  may 
be,  it  exhibits  this  peculiarity  of  two  poles,  one  at  either 
end,  with  a  region  deficient  in  magnetism  between  them. 
One  of  the  poles  of  magnets  is  called  the  positive  (-)-),  and 
the  other  the  negative  ( — )  pole,  while  the  line  joining  the 
poles  is  called  the  axis  of  the  magnet. 

Law  of  Polar  Action. — If  a  permanent  magnet  be 
delicately  balanced  on  a  pivot  at  the  center,  so  that  it  may 
swing  freely,  and  either  pole  of  a  second  magnet  be  suc- 
cessively presented  to  its  two  ends,  it  will  be  found  that 
like  poles  (two  -+-  or  two  —  poles)  repel,  while  unlike  poles 
(a  -f-  and  a  —  pole)  attract  each  other. 


30 


PHYSICAL    GEOGRAPHY. 


Lines  of  Magnetic  Force. — If  one  of  the  poles,  say 
the  -f-  pole,  of  a  strong  magnet,  be  placed  against  the 
lower  side  of  a  horizontal  plate  of  glass,  on  the  upper 
side  of  which  iron  filings  are  scattered,  the  filings  are 
magnetized  by  induction  through  the  glass;  and  if  the 
glass  be  lightly  tapped,  the  filings  tend  to  arrange  them- 
selves in  lines  radiating  from  the  portion  of  the  glass  im- 
mediately over  the  pole  of  the  magnet.  These  are  called 
lines  of  magnetic  force. 


Fig.  8 


This  is  caused  by  the  attraction  of  the  magnet  beneath  the  glass 
for  the  opposite  pole  of  each  iron  filing,  which  has  been  rendered  a 
temporary  magnet  by  induction  ;  hence,  when  the  filings  are  jarred  by 
the  tapping  on  the  glass,  each  points  its  greatest  length  or  axis  away 
from  the  locality  of  the  bar  magnet,  thus  producing  the  radiating 
lines.  These  are  called  lines  of  magnetic  force,  because,  if  a  per- 
manent magnet,  freely  swinging  on  a  pivot  at  its  center,  be  set  upon 
the  glass,  it  will  settle  to  rest  with  its  axis  parallel  with  the  line  or  ray 
beneath  it,  and  its  —  pole  pointed  toward  the  +  pole  of  the  magnet 
beneath  the  glass.  The  same  thing  would  have  happened  if  the 
—  pole  of  the  bar  magnet  had  been  placed  beneath  the  glass,  ex- 
cepting that  in  that  case  the  opposite  (or  -J-)  pole  of  the  swinging 
magnet  would  have  pointed  toward  it.  (Fig.  8.) 

The  earth  itself  is  a  huge  magnet ;  one  of  its  poles 
is  at  present  located  beneath  the  northern  part  of  North 
America,    and    the   other    beneath    the    antarctic   regions 


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3i 


o* 


Fig.  9.— Magnetic  Meridians  in  Northern  Hemisphere. 

south  of  Australia ;  but  these  poles  very  gradually  change 
their  position,  though  they  seem  to  be  confined  to  the 
arctic  and  antarctic  regions  respectively.  The  lines  of 
magnetic  force  of  this  great  terrestrial  magnet  are  usually 
called  magnetic  meridians.  The  direction  of  the  magnetic 
meridians  in  the  northern  hemisphere  is  represented  by  the 
heavy  lines^  in  Figure  9.  Each  of  these  magnetic  merid- 
ians, like  the  lines  of  force  in  the  iron  filings,  indicates  the 
direction  in  which  a  freely  swinging  magnet  will  settle  to 
rest  at  localities  on  that  meridian. 

P.  G.-3. 


32  PHYSICAL    GEOGRAPHY. 

Variation  of  the  Compass. — The  compass  is  essen- 
tially a  freely  swinging  magnet.  The  pole  of  this  magnet 
or  "  needle,"  which  points  to  the  magnetic  pole  in  the  arctic 
regions,  is  called  its  north  or  -f  pole.  But  the  magnetic 
pole  toward  which  the  compass  points,  does  not  coincide 
with  the  end  of  the  earth's  axis  or  true  north ;  hence,  the 
compass  needle  does  not  every-where  indicate  a  true  north- 
and-south  line ;  the  angle  which  it  makes  with  this  line  is 
called  the  variation  of  the  compass. 

The  chart  shows  that  the  variation  is  considerably  west  of  true 
north  over  most  of  the  Atlantic  and  its  coasts,  but  to  the  east  over 
the  Pacific  and  its  coasts.  Possibly  in  consequence  of  a  slow  move- 
ment in  the  magnetic  pole,  the  west  variation  in  the  United  States  is 
now  increasing,  while  the  east  variation  is  diminishing. 

The  cause  of  terrestrial  magnetism  is  undetermined, 
but  it  is  not  improbable  that  it  is  induced  by  electricity 
generated  in  the  luminiferous  ether  by  the  rotation  of  the 
earth.  Magnetic  storms,  or  unusual  movements  of  the 
magnetic  poles  and  meridians,  producing  sudden  vibra- 
tions of  the  compass  needle,  and  sometimes  interfering 
with  the  working  of  telegraph  lines,  occur  occasionally, 
and  are  most  frequent  at  intervals  of  about  eleven  years — 
corresponding  to  times  of  great  disturbance  in  the  sun. 

The  electrical  condition  of  matter  is  rendered  mani- 
fest in  many  ways.  When  in  this  condition  matter  is 
almost  always  heated,  and  sometimes  so  highly  heated  as 
to  become  luminous.  Quite  frequently  the  electrified  body 
becomes  a  temporary  magnet;  sometimes  the  body  is 
thrown  into  sensible  movement ;  sometimes  the  movement 
is  insensible  (molecular),  but  sufficiently  violent  to  over- 
come cohesion,  and  thus  change  the  state  of  aggrega- 
tion of  the  body;  at  other  times,  the  movement  may  be 
violent  enough  to  overcome  chemical  affinity,  and  thus 
cause  the  decomposition  and  entire  disappearance  of  the 
electrified  substance.     Very  frequently  the  collision  of  dis- 


SOME    GENERAL    LAWS    OF    NATURE.  33 

placed  molecules  gives  rise  to  sound.  The  sound  may 
vary  in  intensity,  from  the  scarcely  audible  crack  of  a 
small  electric  spark,  to  the  deafening  crash  of  thunder. 

Development  of  Electricity. — The  expenditure  of 
energy  in  any  manner  and  upon  any  substance  seems  to 
develop  a  greater  or  less  amount  of  electricity;  even  the 
bringing  into  simple  contact  of  any  two  substances  seems 
to  result  in  a  more  or  less  pronounced  electrical  condition 
in  these  substances. 

As  simple  contact  of  two  substances  develops  an  electrical  con- 
dition, this  condition  is  constantly  being  developed  in  nature,  but 
its  effects  are  not  always  seen  because  many  substances  transmit 
electrical  energy  readily  to  the  earth,  where  it  quietly  diffuses  itself. 

Conductors. — All  substances  transmit  electrical  energy, 
but  in  many  substances  its  passage  is  almost  instantaneous ; 
such  substances  are  called  good  conductors  or  conductors. 
Among  them  are  the  metals,  liquids,  and  hence  all  moist 
bodies,  including  living  animals  and  plants.  Non-conductors 
transmit  electrical  energy  with  extreme  slowness ;  such  are 
glass,  dry  air,  stone  and  earthy  substances,  and  dry  ani- 
mal and  vegetable  matter. 

An  electrified  conductor  surrounded  by  non-conductors  retains  its 
electrical  energy  a  long  time,  and  is  said  to  be  insulated.  If  a  piece 
of  brass  be  electrified  and  held  in  the  hand,  its  electrical  energy 
passes  through  the  body,  imparting  a  more  or  less  powerful  shock, 
and  escapes  to  the  earth ;  but  if  the  electrified  brass  rest  on  a  glass 
plate,  and  is  surrounded  by  dry  air,  it  is  insulated. 

Positive  and  Negative  Electricity. — Two  kinds  of 
electrical  energy  always  make  their  appearance  simultane- 
ously when  a  body  is  electrified,  and  are  respectively 
called  positive  and  negative  electricity.  Each  kind  pos- 
sesses a  strong  attraction  for  the  opposite  kind,  but  repels 
electricity  of  the  same  kind.  It  is  convenient  to  conceive 
that  both  kinds  of  electricity  exist  in  all  bodies,  but  when 
present  in  equal  quantities  they  neutralize  each-  other,  and 


34  PHYSICAL    GEOGRAPHY. 

the  body  exhibits  no  electrical  properties.  By  the  appli- 
cation of  energy,  these  electricities  are  conceived  to  be 
separated  and  kept  apart,  the  body  thus  becoming  electri- 
fied, positive  electricity  collecting  at  one  extremity  and 
negative  at  the  other. 

When  a  body  is  electrified,  it  always  electrifies  surrounding 
objects  by  induction  ;  that  is,  the  near  neighborhood  of  an  electrified 
body  causes  a  separation  of  the  two  kinds  of  electricity  in  surround- 
ing objects,  negative  electricity  collecting  on  the  side  of  the  objects 
which  are  nearest  to  the  positive  end  of  the  electrified  body,  and 
vice  versa. 

The  Electric  Spark. — Thus,  there  is  a  strong  attrac- 
tion between  the  electricity  at  either  end  of  the  electrified 
body  and  the  opposite  kind  of  induced  electricity  on  the 
nearest  surfaces  of  surrounding  bodies.  These  electrici- 
ties tend  to  unite,  but  the  intervening  air,  being  a  non- 
conductor, resists  their  union.  If  the  charge  be  suffi- 
ciently intense,  however,  the  electricity  forces  a  passage 
for  itself,  part  of  its  energy  being  transformed,  by  the  re- 
sistance of  the  air,  into  heat.  This  makes  the  air  par- 
ticles along  its  path  white  hot  for  the  fractional  part  of  a 
second,  and  produces  a  streak  of  white  light  through  the 
air  called  an  electric  spark. 

The  more  dense  and  dry  the  intervening  air,  the  greater  must  be 
the  electric  charge  which  is  able  to  penetrate  it,  and  the  more  in- 
tensely luminous  and  streak-like  is  the  resulting  spark.  When  the 
air  is  very  thin  or  rare,  the  passage  through  it  of  an  electric  charge 
produces  a  more  or  less  feeble  glow  rather  than  a  bright  spark,  and 
the  glow  is  often  beautifully  colored.  An  electric  spark  is  generally 
accompanied  by  a  crackling  sound,  more  or  less  audible,  as  the 
spark  is  larger  or  smaller.  The  sound  is  simply  the  clash  of  the  air 
particles  upon  each  other  as  the  air  suddenly  expands  and  contracts 
under  the  great  but  instantaneous  change  of  temperature  produced 
by  the  passage  of  the  electricity. 


PART   I.— THE  EARTH  AS  A  PLANET. 


CHAPTER  I. 

THE    SOLAR    SYSTEM. 


The  heavens  declare  the  glory  of  God;  and  the  firmament  showeth  his  handy* 
work.— Psalm  xix:  i. 

Fixed  Stars  and  Planets. — By  attentively  observing 
the  stars  at  night,  it  will  be  seen  that  they  appear  to 
move  slowly  across  the  sky.  Observations  upon  several 
nights  will  convince  one  that  most  of  the  stars  move  to- 
gether, like  bright  spots  in  a  solid,  revolving  sky.  In 
consequence,  the  position  of  each  star  is  fixed  in  relation 
to  the  others,  and  on  this  account  they  are  called  fixed 
stars.  Occasionally  stars  and  comets  are  seen  whose  ap- 
parent nightly  movement  across  the  sky  is  not  in  unison 
with  that  of  the  others;  they  shift  their  positions  among 
the  fixed  stars  from  night  to  night,  and  are  therefore 
called  planets  (wanderers).  The  sun  is  also  a  wanderer, 
and  appears  each  morning  during  the  year  among  a  dif- 
ferent group  of  the  fixed  stars. 

The  Solar  System. — These  movements  led  astronomers 
to  suspect  that  the  wanderers  have  a  peculiar  relationship 
to  each  other,  and  further  investigation  confirmed  this  sus- 
picion, as  it  is  proved  that  the  planets  and  comets  revolve 
about  the  sun,  thus  forming  a  separate  group  of  heavenly 
bodies,   much  nearer  to   us  than  any  of  the  fixed  stars. 

(35) 


$6  PHYSICAL    GEOGRAPHY. 

We  call  this  separate  group  the  solar  system.  It  is  prob- 
able that  each  of  the  fixed  stars  is  really  a  sun,  and  the 
center  of  a  separate  system  of  planets,  but  so  far  from 
us  that  the  planets  are  invisible. 

The  Sun  is  the  largest  and  most  important  body  in  the 
solar  system.  Its  shape,  like  that  of  all  the  planets,  is 
globular,  or  ball-like,  but  it  differs  from  the  planets  in 
temperature,  its  surface  being  much  hotter  than  the  hot- 
test fire.  The  sun  supplies  the  solar  system  with  radiant 
energy,  which  becomes  sensible  as  heat,  light,  and  in 
other  forms. 

The  sun  has  a  diameter  of  more  than  866,000  miles,  and  weighs 
about  760  times  as  much  as  all  the  planets  put  together.  The  sun  is 
largely  composed  of  matter  in  a  gaseous  state,  many  substances  ex- 
isting there  in  that  form,  with  which  we  are  familiar  only  as  dense 
solids,  such  as  the  metals.  It  is  thought  that  the  heat  of  the  sun  is 
developed  and  maintained  by  the  gradual  contraction  and  conden- 
sation of  its  gaseous  body,  and  to  a  lesser  extent  by  its  collision  with 
very  small  solid  planetary  bodies  called  meteors. 


THE    PLANET    MERCURY 

VENUS 

EARTH 

MARS 

JUPITER 

SATURN 

URANUS 

NEPTUNE 


Fig.  10.— Relative  Diameters  of  the  Sun  and  Larger  Planets. 

The  Planets  are  bodies  much  smaller  than  the  sun, 
around  which,  at  different  distances,  they  revolve,  and 
from  which  they  receive  most  of  their  light  and  heat. 
The  path  of  a  planet  around  the  sun  is  called  its  orbit. 
The  planets  shine  at  night  by  reason  of  the  sunlight  which 
is  reflected  from  their  surface,  while  the  sun  and  the  fixed 
stars  shine  by  their  own  light. 

There  are  more  than  two  hundred  and  fifty  planets,  of  which 
eight  are  vastly  larger  than  the  rest.     The  large  ones  are  named  in 


THE   SOLAR   SYSTEM. 


37 


the  order  of  their  distance  from  the  sun :  Mercury,  Venus,  Earth, 
Mars,  Jupiter,  Saturn,  Uranus,  and  Neptune.  Mercury,  Venus,  Mars, 
Jupiter,  and  Saturn  are  frequently  visible  from  the  earth  as  remark- 
ably large  and  brilliant  stars.  Uranus  is  so  distant  as  to  be  barely 
visible  to  the  naked  eye,  and  Neptune  can  never  be  seen  without  a 
telescope.  Most  of  the  other  planets  are  very  small,  and  lie  between 
the  orbits  of  Mars  and  Jupiter.  They  are  never  visible  to  the  naked 
eye,  and  are  called  planetoids  or  asteroids. 

Satellites. — Several  of  the  planets  are  attended  by  one 
or  more  smaller  bodies  called  satellites,  or  moons,  which 
shine  by  reflected  sunlight,  and  re- 
volve around  their  respective  plan- 
ets, as  the  planets  revolve  around 
the  sun. 

Comets  are  planetary  bodies 
which  revolve  about  the  sun  in  very 
elongated  orbits.  The  mass  of  a 
comet  is  usually  very  small,  but  it 
is  generally  so  widely  distributed 
that  its  volume  is  often  enormous. 
Comets  shine  chiefly  by  reflected 
sunlight,  though  some  comets  ap- 
proach the  sun  so  closely  that  they 
become  sufficiently  heated  to  shine 
by  their  own  light  also. 

Comets  are  often  composed  of  a  com- 

,  n     j  i-j  i  Fig-  ii.— Donati's  Comet, 

paratively  small,  dense,  or  solid  nucleus, 

surrounded  by  a  less  dense  cloud  or  coma.     There  is  frequently, 

though  not  always,  an  extension  of  the  coma  on  one  side  of  the 

comet,  which  forms  the  "tail."     The  tail  is  composed  of  matter  in  a 

state  of  extreme  tenuity,  and  is  often  thousands  of  miles  in  length. 

Meteors.  —  Millions  of  small  fragments  of  mattei, 
possibly  the  debris  of  disintegrated  comets,  revolve  about 
the  sun.  Many  of  them  enter  our  atmosphere,  in  which 
case  the  friction  of  the  air  on  the  rushing   fragment  de- 


38  PHYSICAL    GEOGRAPHY. 

velops  enough  heat  to  ignite  the  fragment  and  render  it 
luminous  as  a  meteor,  ox  "falling  star." 

Meteors  are  generally  entirely  consumed  in  the  air,  but  sometimes 
a  remnant  of  one  reaches  the  earth's  surface  as  a  mass  of  stone  and 
metal  called  an  aerolite.  These  foreign  bodies  show  considerable 
diversities  of  composition ;  but  in  no  case  have  they  yet  revealed 
the  existence  of  any  element  not  found  on  the  earth. 

The  Nebular  Theory. — The  sun  and  all  its  planets 
seem  to  be  composed  of  the  same  kinds  of  matter.  They 
are  all  globular  in  form.  They  all  have  a  spinning  motion 
in  the  same  direction.  The  planets  all  move  around  the 
sun,  and  most  of  the  satellites  around  their  respective 
planets  in  the  same  direction.  The  sun  itself  appears  to 
be  a  very  hot,  gaseous  body,  which  is  gradually  cooling 
and  contracting  in  volume.  These  considerations  have  led 
to  the  nebular  theory  of  the  formation  of  the  solar  system. 
According  to  this  theory,  all  the  matter  composing  the 
various  members  of  the  solar  system  was  once  so  hot  that 
it  existed  as  a  single  enormous  cloud  or  nebula  filling  all 
the  space  within  the  orbit  of  the  most  distant  planet.  As 
the  nebula  cooled  and  contracted,  it  acquired  a  rotary  or 
spinning  motion,  and  threw  off  successive  rings,  each  of 
which,  cooling  and  contracting  about  its  densest  point,  as- 
sumed at  length  the  form  of  a  spinning,  globular  planet, 
revolving  about  the  parent  mass  of  nebula,  which  we  call 
the  sun.  Several  of  the  planets,  in  cooling,  are  supposed 
to  have  thrown  off  secondary  rings,  which  condensed  into 
satellites,  or  moons. 

The  Earth,  upon  which  we  live,  is  one  of  the  eight 
larger  planets, — the  fifth  in  point  of  size,  and  the  third  in 
point  of  distance  from  the  sun.  The  earth  is  attended  by 
a  secondary  planet,  called  the  moon,  which  revolves  about 
the  earth  as  the  earth  revolves  about  the  sun. 

Shape  of  the  Earth. — The  earth  is  nearly  round  or 
globular  in  shape.     If  it  were  exactly  round,   its  shape 


THE    SOLAR    SYSTEM.  39 

would  be  that  of  a  sphere,  but  as  it  is  slightly  flattened  on 
two  opposite  sides,  its  shape  is  that  of  a  spheroid.  If  a 
person  stands  in  the  midst  of  a  vast  plain,  or  on  the  deck 
of  a  vessel  at  sea,  the  surface  of  the  earth  appears  to  be 
flat,  and  stretches  away  in  every  direction  to  a  line  where 
it  seems  to  meet  the  sky;  this  line  is  called  the  horizon. 
The  circular  area  embraced  by  the  horizon  enlarges  as 
the  observer  ascends,  but 
even  from  the  greatest  height 
ever  attained  by  balloonist 
(Fig.  12),  the  visible  portion 
of  the  earth  forms  such  an 
exceedingly  small  propor- 
tion of  its  whole  surface  Flg" l2' 
that  the  curvature  is  entirely  imperceptible. 

But  the  earth  can  not  be  flat,  for  mariners,  by  sailing  continuously 
in  the  same  general  direction,  have  at  last  found  themselves  at  their 

starting-point.     That  the  surface 

>  ^^  w*u  is  curved,  is  proved  by  the  man- 

^ —  """""^^^^         ner  of  disappearance  of  a  ship 

*&r  ^N&  upon  reaching  our  horizon ;  first 

Fi_  I3  the  hull,  or  body,  of  the  vessel 

sinks  out  of  view,  then  the  lower 
sails  disappear,  and  finally  the  highest  parts  of  the  masts  sink  be- 
neath the  horizon.  That  the  curved  surface  is  nearly  that  of  a 
sphere  is  proved  by  the  circular  shadow  which  the  earth  invariably 
casts  on  the  face  of  the  moon  at  the  time  of  a  lunar  eclipse.  Now, 
a  sphere  is  the  only  body  which  can  cast  no  other  than  a  circular 
shadow.  Finally,  very  careful  measurements  upon  the  earth,  and 
observations  upon  the  fixed  stars,  have  proved  conclusively  that  the 
shape  of  the  earth  is  spheroidal — but  very  nearly  spherical — 299  of 
the  shortest  diameters  being  equal  to  298  of  the  longest  diameters. 

Size  of  the  Earth. — The  length  of  the  shortest  diam- 
eter of  the  earth  is  about  7,900  miles.  The  greatest 
diameter  is  about  26  miles  longer.  From  these  diameters, 
it  follows  that  the  greatest  distance  around  the  earth,  or 


(4o) 


EZaSQEI 


THE  SOLAR    SYSTEM.  4 1 

its  circumference,   is  about   25,000  miles,   while  the  total 
area  of  the  earth's  surface  is  197  millions  of  square  miles. 

These  figures  are  much  too  large  to  convey  a  definite  impression. 
The  vastness  of  the  earth  may  better  be  appreciated  by  considering 
that  it  would  take  a  railway  train  moving  a 
mile  a  minute,  17  days  and  nights  of  continuous 
travel  to  complete  the  greatest  distance  around 
it ;  and  that  there  is  room  on  its  surface  for 
fifty-five   countries    as    large    as    the   United 
States.     Large  as  the  earth  seems  to  us,  it  is 
greatly  exceeded  in  size  by  four  of  the  other         Fig.  14.— Relative 
planets,  while  the  surface  of  the  sun  has  more  Areas. 

than  ten  thousand  times  its  area. 

Internal  Temperature  of  the  Earth. — The  occur- 
rence in  many  localities  of  springs  of  hot,  often  boiling, 
water,  and  of  volcanoes  discharging  steam  and  molten 
rock  or  lava,  leads  to  the  belief  that  the  interior  of  the 
earth  is  very  hot.  Observations  in  mines,  wells,  and  deep 
borings  invariably  indicate  an  increase  of  temperature  with 
an  increase  of  depth.  The  rate  of  this  increase  varies 
greatly  in  different  localities,  but  the  average  is  about  i° 
for  every  50  feet.  At  this  rate  of  increase,  a  temperature 
sufficient  to  melt  any  known  substance  would  be  attained 
at  a  depth  of  30  or  40  miles. 

The  Density  of  the  earth  tends  to  confirm  the  belief 
in  a  very  high  internal  temperature.  Calculations  prove 
that  the  earth  weighs  5^  times  as  much  as  a  similar 
globe  of  water.  The  surface  rocks  weigh  from  2^  to  3 
times  as  much  as  water.  The  pressure  to  which  such 
rocks  would  be  subjected  at  great  depths  would  so  greatly 
increase  their  density  that  we  should  expect  the  specific 
gravity  of  the  whole  earth  to  be  much  greater  than  5^. 
There  must  be  some  expansive  force  within,  which  parti- 
ally counteracts  the  pressure.  Heat  is  the  only  force  we 
know  of  that  will  do  this. 


42  PHYSICAL    GEOGRAPHY. 

Condition  of  the  Interior.  —  The  indications  of  a 
great  internal  heat,  together  with  many  facts  in  geology, 
lead  many  to  believe  that  the  earth  is  essentially  a  great 
globe  of  molten  matter,  on  whose  surface  a  cool,  solid 
scum,  or  crust,  has  formed,  which  is  comparatively  thin — 
perhaps  a  hundred  miles  or  more. 
Other  phenomena  have  been  held  to 
indicate  that  the  globe  throughout  is 
as  rigid  as  steel.  Those  who  hold  this 
opinion  believe  it  to  be  possible  that 
the  great  weight  of  the  overlying  rocks 
may  prevent  expansion,  which  accom- 
Fig.  15.-A  crust  100  panies  the  liquefaction  of  all  known 
Miles  Thick.  rocks,   and    thus  retain  the  interior  of 

the  earth  in  a  solid  form,  at  a  temperature  far  above  its 
melting  point.  Whether  the  great  interior  of  the  earth  is 
or  is  not  liquid  by  reason  of  its  heat,  it  seems  certain  that 
the  rocks  at  no  great  depth  are  capable  of  flowing  as  if 
they  were  plastic— J  ike  thick  tar. 

This  peculiarity  would  result  from  inequalities  in  pressure  on  adja- 
cent regions.  The  weight  of  a  few  miles'  thickness  of  the  earth's 
crust  is  great  enough  to  crush  any  known  rock  to  powder,  but  a 
block  of  deeply  buried  rock  can  not  fall  to  pieces  as  powder  because 
of  the  side  pressure  of  adjacent  rocks.  If,  however,  the  pressure  on 
any  side  should  become  less  than  that  on  the  block,  the  latter  would 
be  more  or  less  flattened,  a  portion  of  its  substance  being  forced  into 
the  region  of  diminished  pressure.  Hence,  no  hollow  places,  caves, 
open  cracks,  or  fissures  can  exist  at  great  depths  in  the  earth,  for 
the  enormous  pressure  would  cause  the  adjacent  rocks  to  "creep" 
or  flow  into  the  cavity  and  fill  it 


CHAPTER  II. 

MOVEMENTS    OF    THE    EARTH. 

And  God  said,  Let  there  be  lights  in  the  firmament  of  the  heaven  to  divide  the 
day  from  the  night;  and  let  them  be  for  signs  and  for  seasons,  and  for  days  and 
years. — Genesis  i  :  14. 

Movements  of  the  Earth. — The  earth,  which  seems 
to  us  so  solid  and  immovable,  is  really  in  constant  and 
very  rapid  motion.  It  has  a  spinning  motion,  called  rota- 
tion, on  one  of  its  diameters ;  and  a  much  faster  motion, 
called  revolution,  in  its  orbit  around  the  sun. 

We  can  not  perceive  these  motions  by  observing  objects  in  our 
neighborhood,  because  the  whole  earth  moves  along  smoothly  and 
noiselessly,  carrying  the  atmosphere  and  all  objects  on  its  surface 
along  with  it.  They  thus  preserve  their  relative  positions  as  they 
would  if  the  earth  were  at  rest.  It  was  only  by  carefully  observing 
the  sun  and  the  fixed  stars  that  it  was  discovered  that  their  move- 
ments in  relation  to  the  earth  are  only  apparent,  and  are  caused 
by  the  movements  of  the  earth  itself. 

Rotation. — The   earth   spins,    or  rotates,    at    a   nearly 
uniform  rate  of  speed,  upon  its  shortest 
diameter,  called  its  axis.     The  ends  of 
the  axis  are  called  the  poles.     A   line 
around  the  earth  midway  between  the 
poles  is  called  the  equator.     One  of  the 
results  of  rotation  is  the  succession  of 
day  and  night.     The   sun   shines   upon 
but  one  half  of  the  earth  at  a  time.  The 
other  half,   being  turned    away  from  the  sun,   is  in  dark- 
ness.     As  the  earth  rotates,   each  point   on  its  surface  is 
carried  successively  into  the  light  and  into  the  darkness, 

(43) 


44  PHYSICAL    GEOGRAPHY. 

one  day  and  one  night,  or  about  twenty-four  hours,  mark- 
ing one  complete  rotation. 

As  the  circumference  of  the  earth  is  about  25,000  miles,  and  as 
the  earth  completes  one  rotation  in  twenty-four  hours,  it  follows  that 
a  point  on  the  equator  moves  at  a  speed  of  over  1,000  miles  an  hour. 
The  speed  of  rotation  is  of  course  less  at  points  on  the  surface 
nearer  to  the  poles,  and  at  the  poles  themselves  is  very  slight,  just 
as  in  a  rotating  wheel  a  point  on  the  tire  moves  faster  than  a  point 
on  the  hub. 

Spheroidal  Form  of  the  Earth  caused  by  Rota- 
tion. —The  inertia  of  all  rotating  bodies  gives  them  a  ten- 
dency to  fly  away  from  the  center.  This  tendency  is 
called  centrifugal  force,  and  it  increases  with  the  speed  of 
rotation ;  hence  it  is  greater  at  the  equator  than  toward 
the  poles.  In  obedience  to  this  force,  the  equatorial 
regions  bulge  out  and  the  polar  regions  draw  slightly 
nearer  together,  producing  the  spheroidal  form  of  the 
earth. 

Direction. — The  earth  always  rotates  in  the  same  gen- 
eral direction,  thereby  affording  the  standard  by  which  all 
terrestrial  directions  are  determined.  The  direction  in  which 
the  earth  rotates  is  called  east;  it  is  nearly  that  in  which 
the  sun  first  appears  every  morning.  The  rotation  of  the 
earth  to  the  east  causes  the  sun  and  other  heavenly  bodies 
to  appear  to  move  across  the  sky  in  the  opposite  direc- 
tion ;  this  direction  is  west.  If  we  stand  with  extended 
arms,  facing  east,  our  arms  will  be  parallel  with  the  earth's 
axis.  The  direction  in  which  our  left  arm  points  is  north, 
and  the  end  of  the  axis  in  that  direction  is  called  the 
north  pole.  The  direction  in  which  our  right  arm  points 
is  south,-  and  the  end  of  the  axis  in  that  direction  is  called 
the  south  pole. 

But  the  sun  seldom  rises  exactly  in  the  east,  and  it  is  therefore 
customary  to  reckon  direction  from  the  north,  which  can  be  accu- 
rately determined  by  observations  on  the  North  Star,  or  Polaris,  a 


MOVEMENTS    OF    THE    EARTH. 

fixed  star  situated  almost  directly  over  the  north  pot 
reason  it  is  often  called  the  Pole  Star.  This  star  is  of 
visible  at  places  north  of  the 
equator,  and  can  be  found 
any  clear  night  by  reference 
to  two  stars,  called ' '  the  point- 
ers," in  the  constellation  of 
"The  Dipper."  The  north 
and  south  directions  are  in 
most  places  only  approxi- 
mately indicated  by  the  com- 
pass (page  32). 

Location. — The  earth 
always  rotates  upon  the  Flg'  17' 

same  axis;  hence,  the  ends  or  poles  of  this  axis  mark 
two  fixed  points.  Upon  these  two  points  depends  a  sys- 
tem of  meridians  and  parallels,  by  means  of  -which  the 
location  of  any  point  on  the  surface  of  the  earth  may  be 
exactly  described. 

Meridians  are  lines  conceived  to  be  drawn  on  the 
earth's  surface  directly  from  one  pole  to  the  other.  Their 
direction  is  exactly  north  and  south.  Two  meridians,  ex- 
actly opposite  each  other,  unite  to  form  a  great  circle, 
which  divides  the  earth's  surface  into  an  eastern  and  a 
western  half,  or  hemisphere. 

The  Equator. — The  position  of  the  equator  depends 
upon  the  positions  of  the  poles,  since  it  lies  just  half-way 

between  them.  Its  direction  is 
exactly  east  and  west.  The 
equator  is  also  a  great  circle, 
and  divides  the  earth's  surface 
into  a  northern  and  a  southern 
Fig.  i8.-Meridians  and  hemisphere. 

Parallels.  Parallels  are  lines  conceived 

to  be  drawn  around  the  earth  in  the  same  direction  as  (par- 
allel with)  the  equator.     They  thus  extend  east  and  west. 


46  PHYSICAL    GEOGRAPHY. 

Parallels  divide  the  earth's  surface  into  unequal  parts,  and 
are  called  small  circles. 

Longitude  is  the  angle  which  a  meridian  makes  with 
some  other  meridian,  assumed  as  the  initial,  or  standard, 
meridian.  Any  meridian  may  be  assumed  as  the  standard, 
but  the  one  passing  through  the  observatory  at  Green- 
wich, England,  is  usually  adopted.  Longitude  is  reckoned 
in  degrees  and  parts  thereof,  east  and  west  from  the  stand- 
ard meridian  through  1800  or  half-way  round  the  earth. 
Hence,  the  meridian  of  1800  east  longitude  coincides  with 
the  meridian  of  1800  west  longitude.  Longitude  thus  fixes 
the  position  of  the  meridian  of  any  place  on  the  earth's 
surface  with  respect  to  the  standard  meridian. 

Since  the  meridians  approach  each  other  as  they  near  the  poles, 
the  length  of  a  degree  of  longitude  decreases  from  the  equator, 
where  it  is  ^ihyth  as  long  as  the  equator,  or  69^  miles,  to  the  poles, 
where  it  is  nothing.  The  longitude  of  a  place  is  reckoned  by  ob- 
serving the  difference  of  time  between  that  place  and  the  standard 
meridian.  The  earth  makes  a  complete  rotation,  that  is,  turns  east- 
ward through  3600,  in  24  hours ;  hence,  it  turns  eastward  1 50  in  one 
hour,  or  i°  in  four  minutes.  Therefore,  at  noon  on  a  certain 
meridian  it  is  four  minutes  before  noon  on  a  meridian  i°  to  the  west, 
and  four  minutes  after  noon  on  a  meridian  i°  to  the  east.  Noon 
may  be  approximately  determined  by  observing  when  the  sun 
is  half-way  between  the  eastern  and  western  horizon.  By  com- 
paring a  watch  keeping  the  time  of  the  standard  meridian  with  the 
noon  at  any  place,  thus  determined,  the  dif- 
ference of  time  is  obtained.  If  the  time  by 
the  watch  is  after  noon,  the  longitude  of  the 
place  is  west,  if  before  noon  its  longitude  is 
east,  of  the  standard  meridian,  and  as  many 
degrees  as  4  is  contained  in  the  difference  of 
time  in  minutes. 

Latitude  is  the  angle  included  be- 
tween the  radius  of  curvature  of  the 
earth's  surface  at  any  parallel  and  the  plane  of  the  equator. 
The  angle  LA  T  is  the  latitude  of  parallel  LX.     Latitude 


MOVEMENTS   OF   THE   EARTH. 


47 


Fig.  20. 


is  expressed  in  degrees  and  parts  thereof,  and  is  reckoned 
north  and  south  from  the  equator  to  the  poles,  900  north 
or  south  latitude  corresponding  to  the  north  or  the  south 
pole  respectively.  Latitude  thus  fixes 
the  position  of  any  parallel  with  re- 
spect to  the  equator. 

Degrees  of  latitude  are  68/^  miles  long 
near  the  equator,  but  they  increase  grad- 
ually toward  the  poles  to  a  length  of  69^ 
miles.  The  reason  for  this  slight  increase 
is  the  spheroidal  form  of  the  earth.  The 
convexity  of  its  surface  decreases  from  the 
equator  toward  the  poles ;  therefore,  the 
radius  of  curvature  increases  in  the  same 
direction.  Two  radii  with  the  same  degree 
of  divergence  are  thus  farther  apart  in  polar  than  in  equatorial 
regions,  as  indicated  in  Fig.  20,  in  which  the  spheroidal  form  of 
the  earth  is  greatly  exaggerated.  Latitude  may  be  reckoned  by 
observing  the  angular  distance  between  the  horizon  and  the  celestial 
pole.  The  celestial  poles  are  points  in  the  heavens  directly  over  the 
poles  of  the  earth  or  terrestrial  poles,  the  approximate  position  of 
the  north  celestial  pole  being  marked  by  the  pole  star  Polaris.  At 
the  equator,  where  the  latitude  is  zero,  both  the  celestial  poles  lie  on 
the  horizon.  As  one  travels  from  the  equator  toward  one  of  these 
poles,  the  horizon  sinks  away  from  it,  or  the  celestial  pole  seems  to 
rise  above  the  horizon.  Half-way  from  the  equator  to  the  terrestrial 
pole,  or  in  latitude  450,  the  celestial  pole  is  half-way  to  the  zenith  or 
450  above  the  horizon.  At  the  terrestrial  pole,  in  900  latitude,  the 
celestial  pole  is  overhead,  or  900  from  the  horizon.  Hence,  the 
angular  distance  of  the  celestial  pole  above  the  horizon  is  the  same 
as  the  observer's  latitude  or  angular  distance  from  the  equator. 

Practical  Use  of  Longitude  and  Latitude. — Since 
longitude  fixes  the  position  of  any  meridian  with  respect 
to  the  standard  meridian,  and  latitude  the  position  of  any 
parallel  with  respect  to  the  equator,  it  follows,  if  the  longi- 
tude and  latitude  of  any  place  are  known,  the  position  of 
the  place  on  the  earth's  surface  is  definitely  fixed.  Thus, 
if  a  place  is  in  yj°  03'  W.  long.  Gr.  and  380  53'  N.  lat,  it 

P.  G.-4. 


48  PHYSICAL  GEOGRAPHY. 

is  known  to  be  at  the  intersection  of  the  meridian  yy°  03' 
west  of  Greenwich,  with  the  parallel  380  53'  north  of  the 
equator. 

Revolution. — In  addition  to  rotation,  the  earth  has  a 
forward  movement  through  space  in  its  orbit.  The 
shape  of  the  orbit  is  due  to  the  action  of  two  great  forces. 
These  are:  (1)  the  gravitation  of  the  sun,  which  tends 
to  deflect  the  path  toward  that  luminary;  and  (2)  centrif- 
ugal force,  which  resists  any  deflection  of  the  path  from 
a  straight  line.  As  a  result  of  these  two  forces,  the  orbit 
of  the  earth  becomes  nearly  circular  around  the  sun. 
Earth  The  movement  of  the  earth  in  its  orbit  is 

f*  °    --N        called  its  revolution  to  distinguish  it  from  its 
/     0  movement  of  rotation. 

\        Sun  / 

\..      0rblt   ,/  While  nearly  circular,  the  exact  shape  of  the 

orbit  is  that  of  an  ellipse,  the  sun  being  situated, 
not  at  the  center,  but  at  one  of  the  foci.  Owing  to 
this  fact,  the  distance  from  the  earth  to  the  sun  varies  about  three 
million  miles  in  different  parts  of  the  orbit.  The  mean  distance  is 
about  91^  million  miles, — a  distance  which  a  railroad  train,  moving 
a  mile  a  minute,  would  require  175  years  to  traverse. 

Orbital  Velocity. —  It  takes  about  365  J^  solar  days 
for  the  earth  to  complete  one  revolution  around  the 
sun.  This  interval  of  time  constitutes  a  year.  In  order  to 
traverse  its  orbit  in  365^  days,  the  earth  moves  at  the 
enormous  velocity  of  nearly  1,100  miles  a  minute,  but  its 
speed  is  not  uniform ;  it  moves  faster  when  nearest  the 
sun  (perihelion),  and  slower  when 
most  distant  (aphelion).  ...  rr^^    t 

At  each  point  of  the  orbit,  A  BCD,  the  /       \  /         I 

inertia  of  the  earth  urges  it  in  the  direction        bo         %  yD 

of  the  tangents,  /,  (centrifugal  force,)  while  J\         =  \ 


gravity  draws  it  toward  the  sun,  s.     At  two  "'■■- jo-^ 

points   in  the  orbit,  B,   D,  gravity  acts  at  F*g  M 

right  angles  to  the  centrifugal  force,  and 

Blither  adds  to  nor  diminishes  the  speed,  but  only  bends  the  line  of 


MOVEMENTS   OF   THE    EARTH.  49 

motion  into  a  curve.  At  all  other  points  gravity  either  increases  or 
retards  the  speed ;  thus,  at  A  it  helps  the  earth  forward.  This  in- 
creased speed  adds  to  the  centrifugal  force,  and  not  only  enables  the 
earth  to  resist  being  drawn  into  the  sun,  but,  after  passing  B,  to  in- 
crease its  distance  from  the  sun.  At  C,  however,  gravity  retards  the 
speed  and  diminishes  the  centrifugal  force,  so  that  in  the  end  gravity 
prevails  and  retains  the  earth  in  its  orbit.  The  varying  velocities  of 
the  earth  are  so  nicely  adjusted  to  its  distance  from  the  sun  that  the 
amount  of  heat  it  receives  in  passing  through  equal  angular  dis- 
tances of  its  orbit  are  exactly  equal,  the  greater  velocity  in  perihelion 
just  compensating  the  greater  distance  in  aphelion.  Thus,  the  1800 
of  the  orbit,  xBy,  are  nearer  the  sun,  and  hence  hotter  than  the  1800 
xDy,  but  the  amount  of  heat  received  by  the  earth  is  equal  in  each 
segment,  since  less  time  is  occupied  in  passing  over  the  former  than 
the  latter. 


Plane   of  Ecliptic ^the// 


Fig.  43. 


Inclination  of  the  Earth's  Axis. — The  absolute  di- 
rection of  the  earth's  axis,  at  all  points  of  the  orbit,  is 
nearly  the  same.  This  direction  makes  an  angle  of  about 
66y^°  with  the  plane  of  the  ecliptic,  —  a  plane  passing 
through  the  earth's  orbit  and  the  sun's  center.  The  axis, 
therefore,  leans  about  23^°  (900 — 66%°)  out  of  a  perpen- 
dicular to  the  plane  of  the  ecliptic.  This  angle  constitutes 
the  inclination  of  the  axis. 

Zones. — The  general  distribution  of  the  sun's  heat  over 
the  earth's  surface  has  occasioned  its  division  into  five  belts, 
or  zones,  —  a  torrid,  or  hot  zone,  embracing  the  equatorial 
region ;  two  frigid,  or  cold  zones,  including  the  region  about 
either  pole ;  and  a  temperate  zone  between  the  torrid  zone 
and  either  frigid  zone. 

Any  surface  upon  which  the  sun's  rays  fall  perpendicularly  is 
hotter  than  it  would  be  if  the  rays  fell  obliquely,  because  more  rays 


50 


PHYSICAL   GEOGRAPHY. 


fall  upon  h.     Thus,  ab,  cd,  and  ef  are  equal  spaces,  and  let  r  repre- 
sent the  parallel  rays  of  the  sun  falling  on  them.     It  is  seen  that  cd, 
^^j<t-^=h  upon  which  the  rays  fall  nearly   perpendicu- 

dm  \  larly,    receives    almost   three   times  as  many 

AH  \  rays  as  either  ab  or  ef  where  they  fall  very 

,J||  obliquely.     This   fact  explains  why  morning 

^H  J  and  evening  are  cooler  than  noon,  a  and/  in- 

^^B/^^R  dicating  the  position  of  places  with  reference 

Fis-  24-  to  the  sun's  rays  in  the  morning  and  evening, 

and  cd  the  position  during  the  middle  of  the  day ;  ab  also  indicates 
the  position  of  regions  near  the  poles  of  the  earth,  where  the  sun's 
rays  always  fall  obliquely.  Hence,  these  regions  are  much  colder 
than  those  near  the  equator,  where  the  rays  fall  obliquely  only  in 
the  morning  and  evening. 

The  Tropics  and  the  Polar  Circles, — The  lines 
bounding  the  zones  are  parallels  of  latitude :  those  bound- 
ing the  torrid  zone  are  called  tropics,  and  those  bounding 
the  frigid  zones  are  called  polar  circles.  The  position  of 
these  depends  upon  the  inclination  of  the  earth's  axis. 

In  the  diagram,  ABCD  represents  the  earth  at  four  points  in  its 
orbit  around  the  sun,  S.  At  A  the  sun  at  noon  is  in  the  zenith  of 
the  equator  y"m  Each  day, 
as  the  earth  advances  in  its 
orbit,  the  inclination  of  the 
axis  causes  places  farther 
and  farther  from  the  equator 
to  be  presented  to  the  verti- 
cal rays  of  the  noon  sun, 
until,  at  B,  the  sun  at  noon 
is  vertical  over  z,  which  is 
as  far  from  the  equator,  y, 
as  the  north  pole,  N,  is  dis- 
tant from  x ;  that  is,  23^°,  or  the  angle  of  inclination  of  the  earth's 
axis.  As  the  earth  passes  onward  in  its  orbit,  the  axis  assumes  daily 
a  position  in  relation  to  the  sun  more  nearly  similar  to  that  at  A,  and 
the  earth  presents  to  the  vertical  rays  of  the  noon  sun  points  nearer 
and  nearer  to  the  equator,  until,  at  C,  the  sun  is  exactly  over  the 
equator  again.  Through  the  other  half  of  the  orbit  the  same  phe- 
nomena take   place,  but  on  the  other  side  of  the  equator.    Thus, 


Fig.  a5. 


MOVEMENTS   OF   THE   EARTH.  5 1 

the  only  part  of  the  earth  which  ever  receives  the  vertical  rays  of 
the  sun  lies  between  the  parallels  of  23^°  latitude  on  either  side  of 
the  equator;  hence,  these  parallels  are  taken  as  the  limits  of  the 
hottest  or  "torrid"  zone.  The  parallel  on  the  north  is  called  the 
tropic  of  Cancer,  and  the  one  to  the  south  the  tropic  of  Capricorn. 
They  are  called  tropics  (turnings),  because  over  them  the  sun  ap- 
pears to  turn  and  retrace  his  course  toward  the  equator.  It  will  be 
observed  in  the  diagram  that  when  the  earth  is  at  B  or  D,  the 
region  within  23^°  of  one  of  the  poles  is  in  darkness,  and  hence 
receives  no  light  or  heat  during  an  entire  rotation  of  the  earth. 
These  regions  which,  during  at  least  one  day  of  the  year  receive 
no  heat  rays,  and  during  the  rest  of  the  year  receive  them  only 
very  obliquely,  must  be  the  coldest  parts  of  the  earth ;  hence,  the 
parallels  23^°  from  the  poles,  or  in  latitude  66j^°,  are  taken  as  the 
limits  of  the  frigid  zones.  The  polar  circle  near  the  north  pole  is 
the  Arctic  Circle  ;  that  nearest  the  south  pole  is  the  Antarctic  Circle. 

Length  of  Days  and  Nights. — Owing  to  the  inclina- 
tion of  the  earth's  axis,  there  are  but  two  points  in  the 
orbit  where  the  light  of  the  sun  reaches  both  poles  of  the 
earth  at  the  same  time.  These  two  points,  (A,  C>  Fig. 
25,)  are  called  the  equinoxes  (equal  nights),  for  when  the 
earth  occupies  either  of  these  positions  in  its  orbit,  the 
days  and  nights  are  every-where  of  equal  length.  At  all 
other  points  in  the  orbit,  as  B  and  D,  the  light  of  the 
sun  extends  beyond  one  pole,  but  fails  to  reach  the  other. 
As  a  result,  more  than  half  of  one  polar  hemisphere  is 
illuminated,  and  its  days  are  longer  than  its  nights,  while 
less  than  half  of  the  other  hemisphere  is  illuminated,  and 
consequently  in  that  hemisphere  the  days  are  shorter  than 
the  nights. 

By  referring  to  the  last  diagram,  and  remembering  that  the  earth 
is  constantly  rotating  upon  its  axis  as  well  as  moving  around  the  sun 
in  its  orbit,  it  will  be  seen  that  the  days  and  nights  are  always  of 
equal  length  (12  hours)  at  the  equator,  but  at  all  other  places- they 
are  of  unequal  length,  excepting  when  the  earth  is  at  the  equinoxes. 
The  days  are  shortest  in  the  northern  hemisphere  and  longest  in 
the  southern  when  the  earth  is  at  B.  The  reverse  is  the  case  six 
months  later  when  the  earth  is  at  D.    At  these  positions,  called  sol' 


52  PHYSICAL    GEOGRAPHY. 

stices,  it  is  continuous  day  at  one  polar  circle,  and  continuous  night 
at  the  other,  during  a  complete  rotation  of  the  earth,  or  24  hours, 
while  at  the  poles  it  is  either  continuous  day  or  continuous  night 
while  the  earth  is  passing  from  A  to  C,  or  for  six  months. 

The  Seasons. — The  succession  of  the  seasons  depends 
upon  the  revolution  of  the  earth,  together  with  the  incli- 
nation of  its  axis.  The  earth  is  in  the  position  C  (Fig. 
25)  on  the  2 1  st  of  March.  This  is  the  vernal  equinox,  and 
the  days  and  nights  are  every-where  of  equal  length.  As 
the  earth  moves  forward  in  its  orbit,  the  north  pole  begins 
to  incline  toward  the  sun,  the  days  lengthen  in  the 
northern  hemisphere  and  shorten  in  the  southern;  and, 
since  the  sun  reaches  the  zenith  of  points  north  of  the 
equator,  the  heat  increases  in  the  northern  hemisphere 
while  the  cold  increases  in  the  southern.  After  92^  days, 
or  about  June  21st,  the  sun  reaches  the  zenith  of  the 
tropic  of  Cancer  (D).  This  is  the  summer  solstice  in 
the  northern  hemisphere,  which  receives  the  sun's  rays 
most  perpendicularly,  and  the  winter  solstice  in  the  southern 
hemisphere,  upon  which  the  sun's  rays  fall  most  obliquely. 
As  the  earth  advances,  the  sun  day  by  day  reaches  the 
zenith  of  points  nearer  the  equator,  and  the  days  grow 
shorter  in  the  northern  hemisphere  and  longer  in  the 
southern.  After  92^  days  the  earth  reaches  the  autum- 
nal equinox  (A)  about  September  22.  This  is  the  be- 
ginning of  spring  in  the'  southern  hemisphere.  Moving 
onward,  the  earth  gradually  presents  its  southern  pole  to 
the  sun,  while  the  north  pole  enters  its  annual  period  of 
cold  and  darkness.  For  90  days  the  days  in  the  northern 
hemisphere  grow  shorter  until  the  winter  solstice  is  reached 
about  December  21  (B).  This  is  the  summer  solstice, 
however,  in  the  southern  hemisphere,  the  time  of  its 
longest  day  and  most  direct  exposure  to  the  sun,  which 
reaches  the  zenith  of  the  tropic  of  Capricorn.     Passing  on 


MOVEMENTS    OF   THE    EARTH 


53 


from  this  point  in  its  orbit,  the  earth  gradually  withdraws 
its  south  pole  from  the  sun  until,  after  90  days,  it  reaches 
the  position  of  the  vernal  equinox  again. 

Projections. — It  is  impossible  to  represent  with  perfect 
accuracy  the  curved  surface  of  the  earth  upon  the  flat  sur- 
face of  a  map.  Many  arrangements,  called '  projections y  of 
the  meridians  and  parallels,  have  been  invented,  each  of 
which  reduces  to  a  minimum  some  one  or  more  of  the 
inevitable  inaccuracies,  while  it  may  exaggerate  others. 
When  any  feature  of  the  earth's  surface,  therefore,  is  to 
be  illustrated  by  means  of  a  map,  it  is  best  to  select  from 
the  many  map  projections  one  that  is  designed  to  show 
that  special  feature  most  accurately.  Various  maps  in 
this  book  are  thus  drawn  in  three  different  projections: 
(1)  Mercators,  (2)  Lambert's,  and  (3)  Polar, 

In  Mercator's  Projection  (Fig.  26).  the  meridians  and  parallels 
are  straight  lines  crossing  at  right  angles,  and  the  spaces  be- 
tween them  are  so  proportioned  that  any  continuous  direction  on 
the  earth's  surface  may  be 
represented  by  a  straight 
line  on  the  map.  Hence, 
every  change  in  direction 
of  any  line  on  the  map 
represents  a  corresponding 
change  in  direction  of  the 
line  it  represents  on  the 
earth's  surface.  Therefore, 
this  projection  is  useful  when 
relative  directions  in  differ- 
ent parts  of  the  earth  are  to 
be  compared,  as  the  direc- 
tions of  different  ocean  currents,  etc.  The  scale  of  the  map,  how- 
ever, is  not  uniform,  but  increases  rapidly  and  irregularly  from  the 
equator  toward  the  polar  regions.  Thus,  Greenland  is  represented 
as  larger  than  South  America ;  whereas,  it  is  really  less  than  one 
eighth  as  large. 

In  Lambert's  Projection  (Fig.  27),  equal  areas  on  the  map  repre- 
sent equal  areas  on  the  earth's  surface.     It  may,  therefore,  be  used 


Fig.  26. 


54 


PHYSICAL   GEOGRAPHY. 


when  areas  are  to  be  compared.     The  equator  and  the  central  me- 
ridian (not  drawn  in  Fig.  27)  of  each  hemisphere  are  straight  lines 

drawn  at  right  angles,  but 
the  parallels  and  all  other 
meridians  are  dissimilar 
curved  lines,  the  bounding 
meridians  of  each  hemi- 
sphere joining  at  the  poles 
to  form  circles.  The  space 
between  meridians  de- 
creases toward  the  bound- 
ary of  each  hemisphere,  but 
that  between  parallels  increases  in  such  proportion  that  all  the  sub- 
divisions between  any  two  parallels  have  the  same  area. 

In  Polar  Projection  (Fig.  28),  the  observer  is  supposed  to  be  imme- 
diately over  one  of  the  poles  of  the  earth,  in  the  center  of  the  map, 
and  to  be  able  to  see  at  a  glance  the  whole  polar  hemisphere  from 
the  pole  to  the  equator  (represented  by  the  heavy  circular  line  in  the 
diagram).  The  parallels  and  the  equator  are  indicated  by  concentric 
circles,  and  the  meridians  by  straight  lines  radiating  from  the  pole 
to  the  equator.  This  projection  is  specially  adapted  to  show  the  true 
relative  position  of  features  on 
opposite  sides  of  the  same 
polar  hemisphere.  The  two 
polar  hemispheres  may  be  sep- 
arately shown  in  two  circles 
having  the  north  and  the  south 
poles  respectively  for  their  cen- 
ters, and  each  terminating  at 
the  equator.  The  Isothermal 
and  Isobaric  charts  (pages  63 
and  85)  are  so  drawn  because 
these  features  form  a  complete 
system  in  each  polar  hemi- 
sphere, and  may  therefore  be 
represented  separately.  But 
when  features  to  be  compared  are  not  entirely  embraced  in  one 
polar  hemisphere,  the  whole  surface  of  the  earth  may  be  shown  in 
this  projection,  by  dividing  the  concealed  hemisphere  into  any 
number  of  equal  sectors,  extending  from  the  equator  to  the  pole,  and 
arranging  these  as  points  around  the  equator. 


Fig.  28. 


PART   II.— THE  ATMOSPHERE. 


CHAPTER  III. 
COMPOSITION,    WEIGHT,    AND    HEAT. 

The  Lord  hath  his  way  in  the  whirlwind  and  in  the  storm,  and  the  clouds  are 
the  dust  of  his  feet.— Nahum  i:  3. 

The  Atmosphere  is  the  outer  covering  of  the  earth. 
It  is  composed  of  a  light  and  gaseous  substance  called  air, 
and  completely  envelops  the  solid  and  liquid  parts  of  the 
planet,  filling  the  deepest  depressions,  and  extending  above 
the  tops  of  the  highest  mountains. 

Composition  of  Air. — Air  is  simply  a  mixture  of 
several  elements  and  compounds.  The  most  important  of 
these  are  the  four  invisible  gases,  nitrogen,  oxygen,  water 
vapor,   and  carbonic  acid. 

The  Nitrogen  and  Oxygen  form  the  great  bulk  of  the 
atmosphere  (about  ffths  of  the  whole)  in  the  proportion  of 
four  measures  of  nitrogen  to  one  measure  of  oxygen.  The 
oxygen  is  the  element  useful  to  life.  The  nitrogen  simply 
dilutes  the  oxygen. 

Water  Vapor  is  always  present  in  the  atmosphere.  In 
extremely  cold  air  its  quantity  is  very  minute,  but  in  hot 
air  it  may  form  almost  -g^th  part  of  the  whole.  Vapor  is 
supplied  to  the  atmosphere  by  evaporation  from  moist 
surfaces,  and  is  the  source  of  clouds,  rain,  snow,  hail,  and 
dew. 

CS5) 


56  PHYSICAL   GEOGRAPHY. 

Carbonic  Acid  is  a  compound  of  carbon  and  oxygen. 
It  is  given  off  to  the  atmosphere  in  the  breath  of  animals, 
by  the  decay  of  animal  and  vegetable  matter,  and  by  the 
combustion  of  fuel.  In  amount  it  varies  from  3  to  20 
measures  in  10,000  measures,  but  when  present  in  the 
latter  quantity  the  air  is  unfit  to  breathe,  producing  stupe- 
faction, and  eventually  death.  Plants  are  largely  com- 
posed of  carbon,  which  they  obtain  from  the  carbonic  acid 
of  the  air,  and  thus  prevent  its  undue  accumulation  in  the 
atmosphere. 

In  addition  to  the  above,  there  are  generally  present  in  the 
atmosphere  traces  of  ammonia  and  of  many  other  gases,  besides 
multitudes  of  minute  solid  dust  particles  and  microscopic  living 
germs.  The  dust  motes  are  visible  when  a  ray  of  sunlight  crosses 
a  darkened  room  (page  27). 

Weight  of  Air. — Although  invisible  and  comparatively 
light,  air  is  a  veritable  substance  and  has  appreciable 
weight.  This  is  made  evident  by  the  resistance  it  offers  to 
the  movements  of  an  open  fan,  and,  when  it  is  itself  in 
motion,  by  its  effects  upon  the  sails  of  a  ship  or  a  windmill. 
If  a  hollow  glass  globe  holding  a  cubic  foot  is  emptied 
of  air  and  carefully  weighed  at  sea-level,  when  the  air  is 
again  let  in  it  will  be  found  to  weigh  about  1  J^  ounces 
more  than  before.  This  increase  is  manifestly  the  weight  of  a 
cubic  foot  of  air.  An  equal  bulk  of  water  weighs  about 
840  times  as  much.  It  is  gravitation,  or  the  weight  of  air, 
which  holds  the  atmosphere  to  the  earth,  just  as  it  is  the 
weight  of  water  which  holds  the  oceans  in  their  beds. 

Pressure. — The  weight,  not  of  a  cubic  foot  of  air,  but 
of  the  whole  atmosphere  resting  upon  any  specified  area, 
creates  thereon  a  pressure  called  the  atmospheric  pressure. 

The  Barometer  is  an  instrument  used  to  ascertain  the  amount  of 
atmospheric  pressure.  The  barometer  in  most  common  use  consists 
of  a  glass  tube  about  three  feet  long,  closed  at  the  upper  end  and 
open   below       The   air    being  entirely   taken   from  the  tube,  the 


PRESSURE  OF  THE   ATMOSPHERE. 


57 


open  end  is  immersed  in  a  little  basin  of  mercury  (Fig.  29^).  It  is 
evident  that  the  atmospheric  pressure  upon  the  exposed  surface  in 
the  basin  will  force  a  column  of  mercury  up 
into  the  vacuous  tube,  until  the  weight  of  the 
column  becomes  just  equal  to  that  pressure. 
When  suitably  mounted,  as  in  Fig.  29a,  there 
is  attached  to  the  tube  a  graduated  scale  for 
ascertaining  the  length  of  the  mercurial 
column.  The  length  of  a  column  required  to 
balance  atmospheric  pressure  at  sea-level  is 
found  to  vary  constantly,  but  on  the  average 
it  is  about  30  inches  long ;  and  since  a  column 
of  mercury  one  inch  square  and  30  inches 
long  weighs  14^  pounds,  it  follows  that  this 
is  the  average  weight,  or  pressure,  of  the  at- 
mosphere on  each  square  inch  of  the  earth's 
surface,  at  sea-level. 

Density. — Being  gaseous,  air  is  elas- 
tic; even  a  slight  pressure  squeezes  it 
into  less  space  and  renders  it  denser, 
but  upon  the  removal  of  pressure,  it  im- 
mediately expands  and  becomes  less 
dense.  As  every  portion  of  the  atmos- 
phere sustains  the  pressure  of  the  por- 
tion over  it,  the  lower  part  is  more 
heavily  compressed  than  the  upper.  For 
this  reason  the  atmosphere  is  densest 
near  sea-level,  and  becomes  less  dense 
as  the  distance  above  that  level  in- 
creases. Fig.  29. 

Height. — The  height  or  depth  of  the  atmosphere  has 
never  been  determined.  Observations  with  the  barometer 
indicate  that  with  each  ascent  of  about  3  y2  miles,  one  half 
of  the  former  weight  of  the  atmosphere  is  left  below. 
The  density  decreases,  of  course,  in  the  same  ratio.  At 
a  height  of  7  miles,  the  atmosphere  is  scarcely  dense 
enough  to  sustain  human  life.     At  50  miles  it  is  no  longer 


58  PHYSICAL   GEOGRAPHY. 

dense  enough  to  reflect  the  rays  of  the  setting  sun  to  cause 
twilight.  But  meteors  have  been  observed  at  a  height  of 
200  miles,  and  as  they  are  caused  by  the  rush  of  solid 
bodies  through  the  air,  the  atmosphere,  though  greatly 
rarified,   must  exist  at  that  elevation. 

Since  atmospheric  pressure  decreases  with  an  increase  of  eleva- 
tion, the  mercury  in  a  barometer  falls  as  the  instrument  is  carried 
upward.  To  moderate  elevations,  the  rate  of  this  fall  is  about  ^th 
of  an  inch  for  each  100  feet  of  ascent.  The  barometer  may  there- 
fore be  used  to  determine  the  relative  heights  of  two  or  more  places. 

Uses  of  the  Atmosphere. — Besides  supplying  oxygen 
to  animal,  and  carbon  to  vegetable  life,  the  atmosphere 
contributes  to  the  habitability  of  the  globe  in  three  im- 
portant particulars:  (1)  It  accumulates  the  heat  of  the 
sun  near  the  surface  of  the  planet.  (2)  The  condensation 
of  its  vapor  is  the  only  natural  source  of  the  world's 
supply  of  fresh  water.  (3)  Its  movements,  or  the  winds, 
tend  to  equalize  temperatures  over  the  surface  of  the 
earth,  and  by  its  movements  moisture  is  brought  from  the 
sea  and  distributed  over  the  land. 

Heat  of  the  Atmosphere. — The  sun  may  be  regarded 
as  our  sole  source  of  heat.  The  stars,  the  heated  interior 
of  the  earth,  the  friction  of  meteors  and  of  terrestrial 
bodies,  chemical  combinations,  etc.,  are  all  sources  of 
heat,  but  the  combined  amount  so  produced  is  trifling  in 
comparison  with  that  received  from  the  sun,  and  may  be 
disregarded. 

How  Imparted. — The  heat  of  the  sun  is  imparted  to 
the  atmosphere  in  four  ways: 

(1)  Directly. — In  their  passage  through  the  atmosphere 
the  sun's  rays  lose  about  one  third  of  their  heat.  This  is 
absorbed  by  the  atmosphere  and  raises  its  temperature. 

(2)  Contact. — About  two  thirds  of  the  energy  of  the 
sun's  rays  are  not  absorbed  in  its  passage  through  the  atmos- 


HEAT    OF    THE    ATMOSPHERE.  59 

phere.  This  energy  reaches  the  earth's  surface  and  raises 
its  temperature,  and  evaporates  water.  Contact  with  this 
warmed  surface  warms  the  lower  atmosphere. 

(3)  The  liberation  of  latent  heat  upon  the  condensation 
of  vapor  makes  the  surrounding  air  somewhat  warmer 
than  it  otherwise  would  be. 

(4)  Radiation  from  the  earth's  surface. — The  earth's  sur- 
face, being  warmed  by  the  sun's  rays,  immediately  radiates 
heat  back  toward  space,  but  these  slowly  vibrating  rays 
from  the  earth  have  not  the  penetrative  power  of  the  rays 
of  the  sun,  and  are  largely  absorbed  by  the  lower  atmos- 
phere, which  is  thereby  warmed  (page  21). 

The  atmosphere  thus  causes  heat  to  accumulate  near  the  earth's 
surface ;  it  allows  a  great  portion  of  the  sun's  rays  to  enter,  but  re- 
tards the  escape  of  heat.  Without  this  property  of  the  atmosphere 
no  life  of  any  kind  could  exist  on  the  earth's  surface,  since  its  tem- 
perature, even  under  the  direct  rays  of  a  tropical  sun,  would  proba- 
bly never  rise  above  zero. 

Distribution  of  Temperature.— Since  the  atmosphere 
is  warmed  by  contact  with  and  radiation  from  the  earth's 
surface,  the  lowest  portion  of  the  atmosphere  is  the  warm- 
est; and  the  warmest  part  of  the  lower  atmosphere  is  the 
part  over  the  warmest  portions  of  the  earth,  or  the  por- 
tions in  the  neighborhood  of  the  equator.  There  is,  there- 
fore, a  vertical  and  a  horizontal  variation  of  atmospheric 
temperature. 

Vertical  Variation. —  Many  observations  of  tempera- 
ture made  at  various  elevations  in  different  parts  of  the 
earth,  indicate  that  the  atmosphere  grows  colder  at  the 
average  rate  of  i°  Fahrenheit  for  each  300  feet  of  increased 
elevation. 

Horizontal  Variation. — Since  the  sun  is  vertical  over 
the  northern  hemisphere  in  our  summer,  and  over  the 
southern  hemisphere  in  our  winter,  the  amount  of  heat 
received  by  either  hemisphere  during  the  two  seasons  is 


6o  PHYSICAL   GEOGRAPHY 

very  different.  In  both  hemispheres  the  temperature  de- 
creases from  equatorial  toward  polar  regions,  but  the  de- 
crease is  much  more  rapid  on  some  meridians  than  it  is  on 
others. 

This  irregularity  is  caused  by  the  different  effects  of 
heat  upon  land  and  water,  owing  to  their  differences  (i)  in 
specific  capacities  for  heat,  (2)  in  penetrability  for  heat 
rays,  and  (3)  in  state  of  aggregation,  one  being  a  solid 
and  the  other  a  liquid. 

Specific  Heat. — It  has  been  stated  (page  23)  that  the 
same  amount  of  energy  produces  different  changes  of 
temperature  in  different  substances.  Now,  a  water  surface 
requires  nearly  twice  as  much  energy  to  raise  its  tempera- 
ture by  a  given  amount  as  an  equal  area  of  land ;  that  is, 
if  equal  surfaces  of  land  and  water,  at  the  same  tempera- 
ture, are  equally  exposed  to  the  rays  of  the  sun,  the  land 
will  be  warmed  nearly  twice  as  much  as  the  water. 

Penetrability. — The  solar  rays  can  not  penetrate  deeply 
into  the  solid  land ;  and,  as  land  is  a  very  poor  heat 
conductor,  all  the  sun  heat  received  is  confined  to  a 
thin  surface  stratum.  This  stratum  is  thus  quickly  and 
strongly  warmed,  and  heats  the  overlying  air  in  contact 
with  it.  But  during  the  night,  when  the  source  of  heat  is 
withdrawn,  the  thin  surface  stratum  of  the  land  and  its 
overlying  air  lose  their  heat  by  outward  radiation  with 
almost  equal  rapidity.  Solar  rays  affect  water,  however, 
to  a  depth  of  about' 500  feet,  and  warmth  is  thus  distrib- 
uted throughout  a  comparatively  thick  layer,  whose  tem- 
perature, therefore,  does  not  rise  so  high  as  that  of  the 
thin  stratum  of  land.  During  the  night  the  water  surface 
cools  more  slowly  than  the  land,  for  as  soon  as  the  surface 
becomes  cooled  in  the  slightest  degree  it  contracts,  be- 
comes heavier,  and  sinks,  being  replaced  by  the  warmer 
and  lighter  water  from  beneath. 


HEAT    OF    THE    ATMOSPHERE.  6 1 

The  difference  between  the  temperature  of  land  and  water  is  in- 
creased by  the  evaporation  from  the  water  surface,  sensible  heat  of 
the  water  and  overlying  air  becoming  latent.  Fully  one  half  the 
sun  heat  falling  upon  the  oceans  is  thus  rendered  insensible,  while 
none  of  the  heat  falling  on  dry  land  becomes  latent.  Much  of  the 
heat  rendered  latent  at  the  surface  of  the  ocean  is  liberated  over  the 
land  on  the  condensation  of  the  vapor  into  clouds  and  rain,  and 
thus  warms  the  upper  land  air  at  the  expense  of  the  lower  ocean  air. 
Another  reason  why  the  sea  is  heated  and  cooled  more  slowly  than 
the  land,  is  that  the  ocean  air  generally  contains  more  moisture 
than  land  air.  The  more  moisture  air  contains  the  more  impene- 
trable it  is  to  solar  rays  (page  22).  The  air  over  the  sea,  therefore, 
stops  and  radiates  back  more  of  the  entering  sun  heat  during  the 
day  and  more  of  the  escaping  surface  heat  during  the  night  than 
does  the  drier  land  air. 

State  of  Aggregation. — The  solid  land  is  stationary, 
and  retains  or  radiates  its  heat  in  the  same  place  where  it 
is  received.  Water,  however,  is  susceptible  of  being 
moved  in  currents,  and  thus  of  receiving  heat  in  one 
place  and  losing  it  in  another.  Ocean  currents  carry  warm 
water  toward  the  poles  and  return  cold  water  to  the 
equator;  thus  nearly  one  half  the  heat  received  by  the 
torrid  zone  is  conveyed  into  higher  latitudes. 

Isothermal  Charts. — If  upon  a  map  all  places  having 
the  same  mean  temperature  are  connected  by  lines,  such 
lines  are  called  isothermal  lines  or  simply  isotherms.  Taken 
collectively,  these  lines  indicate  the  distribution  of  mean 
temperature  over  the  region  embraced  in  the  map.  Such 
a  map  is  called  an  isothermal  map  or  chart.  On  the  ac- 
companying isothermal  charts  the  regions  in  which  the 
temperature  is  higher  than  700  are  tinted  pink,  those  in 
which  it  is  lower  than  300  are  tinted  blue,  while  those 
having  a  temperature  between  300  and  jo°  are  untinted. 

Northern  Hemisphere  in  Winter. — It  will  be  seen  on 
the  chart  that  in  temperate  and  polar  regions  the  land  air 
is  colder  than  the  sea  air  in  corresponding  latitudes,  the 


62  PHYSICAL    GEOGRAPHY. 

isotherm  of  300  Fahr.  descending  to  the  neighborhood  of 
400  latitude  over  the  land,  while  over  the  sea  this  isotherm 
lies  in  much  higher  latitudes,  for  the  land  at  this  season 
loses  more  heat  by  radiation  during  the  long  nights  than  it 
receives  during  the  short  days,  but  the  sea  loses  less  heat 
by  radiation  and  is  constantly  receiving  heat  by  warm  cur- 
rents from  the  equator.  The  difference  in  temperature  is 
greater  near  the  parallel  of  6o°  than  in  any  other  latitude, 
amounting  to  about  470  Fahr.  In  equatorial  regions  the 
land  air  is  slightly  warmer  than  the  sea  air;  thus,  a  tem- 
perature of  more  than  8o°  Fahr.  prevails  over  equatorial 
Africa  and  South  America,  while  a  temperature  of  less 
than  8o°  Fahr.  prevails  over  equatorial  oceans,  for  currents 
carry  heat  away  from  the  seas  of  these  regions,  while  the 
diurnal  loss  and  gain  of  heat  are  about  equal  on  the  land, 
since  the  nights  and  days  are  of  nearly  equal  length 
throughout  the  year. 

Northern  Hemisphere  in  Summer. — During  the 
long  days  of  summer  the  land  receives  more  heat  than 
it  radiates  during  the  short  nights,  and  thus  accumu- 
lating heat,  becomes  in  all  latitudes  warmer  than  the  sea 
surface. in  corresponding  latitudes.  The  difference  in  tern 
perature  is  not  great,  however,  because  the  sea  absorbs 
heat  by  day  as  well  as  the  land ;  and  besides,  in  higher 
latitudes,  it  is  constantly  receiving  heat  in  warm  currents 
from  lower  latitudes.  The  difference  amounts  to  about 
1 8°  Fahr.  near  the  parallel  of  6o°,  and  to  but  250  Fahr. 
where  it  fo  greatest,  —  near  the  parallel  of  400. 

In  the  Southern  Hemisphere,  the  distribution  of  tem- 
perature is  seen  to  be  much  more  regular  than  in  the 
northern  hemisphere,  because  its  surface  is  more  nearly 
uniform,  being  almost  entirely  water  beyond  300  south 
latitude.  The  only  considerable  irregularity  in  the  isotherms 
occurs   ia   tropical   regions,   in   the   neighborhood   of  the. 


ISOTH  ERM  A  L 
CH  ARTS  OF 


64  PHYSICAL    GEOGRAPHY. 

land  surfaces.  Here,  as  in  corresponding  regions  in  the 
northern  hemisphere  and  for  like  reasons,  the  land  is 
slightly  warmer  than  the  sea  surface  at  all  seasons. 

General  Deduction. — It  is  thus  seen  that  the  water 
surface  is  not  warmed  so  greatly  during  the  day  or  during 
the  summer,  nor  is  it  cooled  so  much  at  night  or  in  winter 
as  the  land  surfaces;  therefore,  a  water  surface  tends  to 
preserve  throughout  the  year  a  uniform  temperature  in  its  over- 
lying air,  while  the  air  over  the  land  may  become  both  ex- 
tremely hot  and  extremely  cold. 

The  Thermal  Equator. — The  line  along  which  the 
greatest  heat  on  the  earth's  surface  occurs  is  called  the 
thermal  equator.  As  the  sun  is  nearly  vertical  over  the 
southern  tropic  in  January,  and  over  the  northern  tropic 
in  July,  it  might  be  expected  that  during  the  year  the 
thermal  equator  would  travel  backward  and  forward  with 
the  sun  between  these  parallels.  The  different  powers  of 
land  and  water  for  accumulating  and  retaining  heat,  how- 
ever, greatly  modify  its  annual  journey.  In  July  (page  63) 
the  high  temperature  of  the  great  land  surfaces  carries  the 
thermal  equator  to  between  200  and  300  north  latitude 
over  the  continents,  while,  in  consequence  of  the  slower 
change  of  temperature  of  the  water  surfaces,  it  lies  in  the 
neighborhood  of  only  io°  north  latitude  over  the  oceans. 
In  January,  the  summer  of  the  southern  hemisphere  (page 
65),  the  influence  of  the  larger  land  masses  to  the  north  is 
still  great  enough  to  hold  the  thermal  equator  very  near 
the  geographical  equator  in  the  southern  hemisphere, 
while  the  western  extensions  of  Africa  and  South  America 
prevent  it  from  crossing  into  the  southern  hemisphere  at 
all  in  those  regions. 


CHAPTER  IV. 

MOISTURE    OF   THE    ATMOSPHERE. 

All  the  rivers  run  into  the  sea  ;  yet  the  sea  is  not  full :  unto  the  place  from 
whence  the  rivers  come,  thither  they  return  again. — Ecclesiastes  i  :  7. 

Source. — The  atmosphere  obtains  its  moisture  by  the 
process  of  evaporation  from  all  the  moist  surfaces  of  the 
earth,  but  mostly  from  the  great  moist  surface  which 
covers  three  quarters  of  the  globe — the  sea. 

Vapor. — Water  ceases  to  be  a  liquid  upon  evapora- 
tion, and  enters  the  atmosphere  as  a  gas  called  vapor. 
Vapor  is  transparent,  and  hence  invisible.  It  is  only  after 
the  vapor  of  the  atmosphere  condenses  into  a  liquid  (or 
solid)  form  that  it  becomes  visible ;  as  vapor  it  can  never 
be  seen. 

Effect  of  Temperature. — A  volume  of  air  at  a  given 
temperature  can  hold  only  a  certain  quantity  of  vapor; 
if  this  air  be  warmed,  it  can  hold  a  greater  quantity  of 
vapor ;  if  it  be  cooled,  its  capacity  for  vapor  decreases.  A 
cubic  foot  of  air  at  a  temperature  of  zero  (Fahrenheit)  can 
hold  only  half  a  grain  of  vapor ;  at  a  temperature  of  3 2° 
it  can  hold  more  than  2  grains ;  at  a  temperature  of  6o°  it 
can  hold  5^  grains;  at  a  temperature  of  900  it  can  hold 
almost  15  grains,   etc. 

Saturated  Air. — When  air  at  any  given  temperature 
contains  all  the  vapor  it  can  hold  at  that  temperature,  it  is 
said  to  be  saturated.  If  air  which  is  not  saturated  comes 
in  contact  with  a  moist  surface,  it  may  evaporate  water 
until  it  becomes  saturated.     If  saturated  air  is  cooled,  it 

(66) 


MOISTURE    OF    THE    ATMOSPHERE.  6j 

can  no  longer  hold  all  of  its  vapor;  a  portion  of  it,  there- 
fore, condenses  into  very  small  globules  of  water,  or  (if 
the  temperature  be  low  enough)  into  minute  crystals  of 
ice,   and  becomes  visible. 

Effect  of  Evaporation  and  Condensation. — The  im- 
mediate effect  of  evaporation  is  to  make  all  bodies  in  the 
immediate  vicinity  colder,  or  to  retard  their  growing 
warmer,  sensible  heat  being  abstracted  from  these  bodies 
and  converted  into  latent  heat.  Condensation  warms  sur- 
rounding bodies,  or  retards  their  cooling,  since  the  latent 
heat  again  becomes  sensible  heat  as  the  vapor  passes  into 
the  liquid  or  solid  form. 

General  Distribution  of  Vapor, —  Evaporation  and 
condensation  are  constantly  going  on  in  nature,  and  there- 
fore the  amount  of  vapor  in  the  atmosphere  is  constantly 
changing ;  but  as  warm  air  has  a  greater  capacity  for  vapor 
than  cold  air,  it  is  generally  true  that  the  amount  of  vapor 
in  the  air  decreases  from  the  surface  of  the  earth  upward, 
and  from  the  equator  toward  the  poles.  It  is  estimated 
that  almost  one  half  the  vapor  in  the  atmosphere  occurs 
lower  than  a  height  of  one  mile  from  the  sea-level,  and 
that  fully  nine  tenths  occur  lower  than  four  miles. 

Relative  Humidity. —  Since  the  capacity  of  air  for 
vapor  varies  so  rapidly  with  temperature,  the  absolute 
humidity,  or  amount  of  vapor  present,  gives  no  definite 
idea  of  the  dampness  of  the  air,  for  the  amount  of  vapor 
which  saturates  air  at  6o°  temperature  and  makes  it  feel 
very  damp,  is  but  little  more  than  one  third  of  the 
amount  required  to  saturate  air  at  900  temperature;  with 
this  amount  of  vapor  present,  air  at  the  latter  temperature 
feels  excessively  dry  and  evaporates  water  with  avidity. 
It  is  therefore  common  to  determine  the  proportion  which 
the  vapor  present  at  any  temperature  forms  of  the  amount 
which  would  saturate  the  air  at  that   temperature.     This 


68  PHYSICAL    GEOGRAPHY. 

is  called  the  relative  humidity  of  the  air.  Thus,  if  the  rel- 
ative humidity  is  25^,  50%,  or  7$%,  the  air  contains  %} 
y2,  or  y±  of  the  vapor  it  is  capable  of  holding  at  its  tem- 
perature. Since  air  loses  its  capacity  for  vapor  by  cool- 
ing, it  follows  that  when  air  is  cooled  its  relative  humidity 
increases,  until,  when  cooled  to  the  point  of  saturation, 
its  relative  humidity  is  100.  Any  further  cooling  would 
produce  condensation.  Thus,  since  temperature  decreases 
as  the  elevation  above  the  earth's  surface  increases,  evap- 
oration may  be  taking  place  in  the  lower  part  of  a  mass 
of  air,  while  condensation  is  in  progress  in  the  upper  part. 

The  relative  humidity  is  determined  by  means  of  an  instrument 
called  a  Hygrometer.  The  hygrometer  in  most  common  use  consists 
of  two  ordinary  thermometers,  the  bulb  of  one  of  which  is  covered 
by  a  small  piece  of  cloth  kept  constantly  moist.  The  evaporation 
from  this  moist  surface  mak«s  the  bulb  it  covers  colder  than  the 
other  bulb,  and  the  two  thermometers  register  different  temperatures. 
If  the  air  is  dry,  evaporation  is  rapid  and  this  difference  is  great ;  if 
the  air  is  moist,  evaporation  is  slower  and  the  difference  in  tempera- 
ture is  less.  Tables  have  been  prepared  from  which  the  relative 
humidity  corresponding  to  each  degree  of  these  differences  between 
the  "  wet  and  the  dry  bulb  thermometers,"  at  any  temperature,  may 
at  once  be  obtained. 

Mist  or  Fog  is  a  vast  multitude  of  minute  globules 
of  water  in  the  air  near  the  earth's  surface.  Fog  may  be 
produced  by  the  spray  thrown  off  by  falling  or  otherwise 
violently  agitated  water,  but  it  is  usually  caused  by  the 
cooling  of  saturated  air,  and  the  consequent  condensation 
of  a  portion  of  its  vapor. 

In  winter,  when  our  warm,  moist  breath  passes  from  the  mouth 
into  the  cold  air,  it  is  chilled,  and  a  portion  of  its  vapor  condenses 
into  a  visible  mist.  Mists  frequently  form  over  sheets  of  water  in 
summer  nights,  because  the  neighboring  land  cools  at  night  faster 
than  the  water,  and  thus  cools  the  atmosphere  in  contact  with  it ; 
this,  in  turn,  chills  the  moist  air  over  the  water  below  its  point 
of  saturation,  causing  part  of  its  vapor  to  condense  into  a  mist. 
When  the  heat  of  the  sun  in  the  morning  increases  the  capacity  of 


MOISTURE    OF   THE    ATMOSPHERE.  69 

the  air  for  vapor,  the  mist  evaporates  and  disappears.  Mountain 
tops  are  frequently  enveloped  in  mist  because  air  currents,  striking 
the  mountain  sides,  are  forced  up  the  slopes  into  higher  regions  of 
the  atmosphere,  and  thereby  chilled  below  their  point  of  saturation. 
The  solrd  particles,  or  dust  motes,  in  the  air  are  great  promoters  of 
the  formation  of  fogs,  since  they  may  radiate  heat  faster  and  thus 
become  colder  than  the  surrounding  air,  which  is  slightly  cooled  by 
contact  with  them.  When  this  is  the  case,  it  is  probable  that  each 
mist  globule  is  formed  around  a  dust  mote. 

Clouds  are  merely  fogs  formed  at  some  distance  above 
the  earth's  surface.  Clouds  may  be  formed  by  radiation 
between  warm  and  cold  currents  of  air,  but  the  chief 
causes  of  their  formation  are  the  mechanical  cooling  of  an 
ascending  current  of  air,  and  the  cooling  by  radiation  of  a 
poleward- moving  current  of  air. 

As  air  ascends  and  is  relieved  of  a  portion  of  atmospheric  pres- 
sure, it  expands,  and  pushes  aside  the  surrounding  air.  In  thus  do- 
ing work,  some  of  its  energy  must  be  expended;  that  is,  the  velocity 
of  its  molecules  is  decreased,  and  it  is  cooled.  Therefore,  when  air 
ascends  it  becomes  constantly  cooler.  The  reverse  occurs  when  air 
descends ;  the  air  is  compressed  by  the  increased  atmospheric  pres- 
sure, and  work  is  done  upon  it,  whereby  the  velocity  of  its  molecules 
is  increased,  and  the  air  becomes  warmer.  Until  its  point  of  satura- 
tion is  reached,  ascending  air  is  thus  cooled  i°  for  each  183  feet  of 
ascent,  but  saturated  air  is  cooled  more  slowly,  owing  to  the  effect  of 
the  liberation  of  latent  heat  on  the  condensation  of  its  vapor.  As 
descending  air  grows  warmer  its  vapor  does  not  condense,  and  there- 
fore both  dry  and  moist  air  grow  i°  warmer  for  each  183  feet  of 'descent. 

Height  of  Clouds, — Since  most  of  the  vapor  occurs 
in  the  lower  part  of  the  atmosphere,  clouds  are  most 
common  at  no  considerable  altitude.  The  mean  elevation 
of  clouds  in  the  temperate  zones  is  about  one  half  a  mile, 
while  the  highest  clouds  ever  seen  are  probably  within  ten 
miles  of  the  earth's  surface. 

For  convenience  of  description,  clouds  have  been  divided  into 
three  great  classes,  which  are  named  from  their  general  shapes: 


7° 


PHYSICAL   GEOGRAPHY. 


Cirrus. 


Stratus.         -fc-v 


r       Cumulus. 
Fig.  30.— Classes  of  Clouds. 


Nimbus. 


cirrus,   or  feathery   clouds ;    cumulus,    or   heaped-up   clouds ;    and 
stratus,  or  spread-out  clouds. 

(1)  Cirrus  are  the  highest  of  all  clouds.  They  are  seen  in  fair 
weather  as  little,  white,  feathery  patches  in  the  blue  sky.  These 
clouds  are  so  high  that  their  temperature  must  be  below  the  freezing 
point,  and  they  are  consequently  thought  to  consist  of  minute  ice 
crystals. 

(2)  Cumulus  are  the  familiar,  dome-shaped  masses  of  cloud  hav- 
ing generally  nearly  horizontal  bases.  They  are  formed  by  ascend- 
ing currents  of  air,  the  horizontal  base  of  the  cloud  marking  the 
altitude  where  the  decreasing  temperature  begins  to  condense  the 
vapor  of  the  ascending  air. 

(3)  Stratus  are  the  continuous,  horizontal  layers  of  cloud,  of 
general  uniform  thickness.  They  are  the  lowest  clouds,  and  fre- 
quently appear  in  the  morning  and  evening  of  fine  days  as  a  low, 
foggy  canopy  overspreading  the  whole  or  a  part  of  the  sky,  and 
disappearing  as  the  heat  of  the  day  increases.     All  low,  detached 


MOISTURE   OF  THE   ATMOSPHERE.  ?1 

clouds  which  look  like  lifted  fog  and  are  not  consolidated  into  defi- 
nite form,  are  stratus  clouds. 

By  the  various  combinations  of  the  three  principal  classes  of 
clouds  are  obtained  the  cirro-stratus,  or  "  Noah's  ark  "  clouds ;  cirro- 
cumulus,  or  "  a  mackerel  sky  "  ;  cumulo-stratus \  or  rain-threatening 
clouds;  and  nimbus,  or  the  rain-cloud  proper. 

Clouds  are  spoken  of  as  suspended  in  the  air, 
but  their  globules  are  generally  descending  slowly  through 
the  force  of  gravity.  They  generally  do  not  descend  far, 
however,  before  they  reach  warmer  regions  of  the  atmos- 
phere, where  the  lower  portion  evaporates  and  disappears. 
This  accounts  for  the  rapid  change  usually  observed  in  the 
shape  of  clouds.  Some  portions  of  the  cloud  are  disap- 
pearing by  evaporation,  while  other  parts  are  forming  by 
condensation. 

One  of  the  chief  uses  of  clouds  is  the  assistance 
they  render  in  maintaining  an  equable  temperature  at  the 
earth's  surface.  In  its  liquid  form,  moisture  obstructs  the 
passage  of  heat  rays  much  more  than  in  its  vaporous 
form.  Clouds,  therefore,  stop  much  of  the  sun's  heat, 
and  so  prevent  the  earth  from  becoming  too  hot  during 
the  day-time ;  while,  by  absorbing  and  radiating  back  a 
portion  of  the  heat  which  is  constantly  streaming  off  from 
the  earth,  they  prevent  its  surface  from  becoming  too  cold 
at  night.  This  is  the  reason  why  cloudy  days  are  gener- 
ally cooler,  but  cloudy  nights  warmer,  than  fair  ones. 

Rain. — When  a  cloud  is  of  considerable  thickness,  and 
the  air  beneath  is  nearly  saturated,  the  globules  in  their 
gradual  descent  through  the  cloud  unite  to  form  larger 
drops,  which,  acquiring  greater  weight  with  their  increase 
in  size,  descend  faster  than  they  evaporate;  and,  if  the 
temperature  be  above  the  freezing  point,  may  finally  reach 
the  earth  as  rain. 

Rain-water. — When  water  evaporates,  all  impurities  are 
left  behind;    vapor,    therefore,   condenses  into  absolutely 


J  2  PHYSICAL   GEOGRAPHY. 

pure  water ;  but  all  the  gases  which  compose  the  air  are 
soluble  in  water,  and  hence  rain-water,  when  it  reaches  the 
earth,  is  never  pure,  being  always  more  or  less  impreg- 
nated with  these  gases,  and  containing  besides  dust  motes 
and  other  solid  particles  which  it  has  picked  up  in  its  de- 
scent through  the  air. 

Uses  of  Rain. — Rain,  therefore,  in  addition  to  supply- 
ing the  rivers,  springs,  and  wells  of  the  earth  with  water, 
performs  an  important  office  in  washing  and  purifying  the 
air  and  rendering  it  more  healthful. 

Snow. — If  the  temperature  of  a  cloud  is  below  the  freez- 
ing point,  the  cloud  is  composed  of  minute  ice  crystals 
instead  of  water  globules.  If  the  air  beneath  such  a  cloud 
is  nearly  saturated  and  of  sufficiently  low  temperature,  the 
ice  crystals  of  the  cloud  accumulate  in  their  descent  into 
flakes,  which  may  reach  the  earth.  We  call  this  phenom- 
enon snow. 

Shape  of  Snow-flakes. — When  snow-flakes  are  formed 
in  calm  air,  they  arrange  themselves,  according  to  the 
laws  of  the  crystallization  of  water,  into  little  six-sided 
plates  or  six-pointed  stars.  Although  over  a  thousand 
different  shapes  have  been  observed  in  snow  crystals,  each 
shape  adheres  to  the  general  law  of  six-sidedness. 

Sleet. — When  driven  about  by  wind,  the  flakes  lose 
this  delicate  arrangement,  and  when  the  temperature  is 
such  that  the  snow  reaches  the  ground  in  a  partly  melted 
condition,  it  is  called  sleet.  In  the  interior  of  continents 
the  ground  in  winter  is  usually  colder  than  the  air,  and 
the  sleet  upon  reaching  it  immediately  freezes,  incasing 
the  ground  and  vegetation  in  a  coating  of  clear  ice.  This 
seldom  happens  on  coasts  and  islands,  where  the  moister 
air  prevents  the  excessive  cooling  of  the  ground.  In  such 
localities,  the  continued  melting  of  the  sleet  in  the  lower 
air  gives  it  the  appearance  of  a  fine,  driving  rain. 


MOISTURE   OF   THE   ATMOSPHER] 


*& 


■*- 


Snow- storms  are  more  frequent,  and  the  snowfall  is 
heavier  when  the  temperature  is  near  the  freezing  point 
than  when  it  is  much  colder,  because  the  colder  air  has  so 
slight  a  capacity  for  vapor  that  it  can  yield  but  very  little 
moisture  to  form  snow. 

Snow  Line. — Since  the  atmosphere  grows  colder  with 
increase  of  elevation,  there  must  be  some  altitude  where 
the  temperature  seldom  rises  above  the  freezing  point, 
even  in  summer.     Above  this  altitude,   the  moisture  of 


Fig.  31.— Some  Shapes  of  Snow  Crystals. 


the  air  is  usually  precipitated  in  the  form  of  ice  or  snow, 
and  if  the  precipitation  is  moderately  heavy,  the  snow 
never  entirely  disappears  from  the  ground.  The  lower 
limit  of  this  region  of  perpetual  snow  is  called  the  snow 
line.  The  snow  line  is  higher  in  equatorial  than  in  polar 
regions. 

The  mean  altitude  of  the  snow  line  at  the  equator  is  about  16,000  ft. 
In  the  mountains  of  Spain,  370  N.  lat.,  its  altitude  is  about  11,000  " 
In  the  Swiss  Alps,    .     .    .     47°  N.  lat.,  "        "        "      "       9,000  " 

In  Norway, 620  N.  lat.,  "         "        "       "        5,000 " 

In  Lapland, 690  N.  lat.,  "         "        ••       "       3,300" 

In  Baren  Island,       ...     75°  N.  lat.,  "         "        "       "  600  " 

In  Spitzbergen,    ....     8o°  N.  lat.,  it  sinks  nearly  to  sea-level. 


74  PHYSICAL    GEOGRAPHY. 

Uses  of  Snow. — One  of  the  chief  uses  of  snow  arises 
from  the  fact  that  it  is  a  very  poor  conductor  of  heat. 
A  layer  of  snow  in  winter  acts  as  a  blanket,  preventing 
the  loss  of  the  earth's  heat  by  radiation,  and  keeping  the 
ground  soft  and  moist;  but  the  soil  and  vegetation  left  ex- 
posed lose  their  heat  by  radiation,  and  are  frozen  hard 
and  stiff. 

Hail. — Pellets  of  ice  falling  in  shower  are  called  hail. 
These  pellets,  or  hailstones,  vary  from  the  size  of  small 
shot  to  that  of  hens'  eggs.  Hailstones  are  sometimes 
composed  throughout  of  clear  ice,  but  usually  there  is  a 
nucleus  of  hard,  compact  snow,  surrounded  by  alternate 
layers  of  ice  and  snow.  Hail  is  more  common  in  summer 
than  in  winter,  and  in  hot  than  in  rool  weather.  It  fre- 
quently precedes  or  accompanies  a  thunder  shower. 

Hail  is  now  believed  to  be  caused  by  the  rapid  ascent  and  con- 
sequent rapid  cooling  of  quite  warm,  moist  air.  Below  a  certain 
height  the  vapor  condenses  into  cloud  and  rain,  but  above  that 
height  into  snow.  The  rain  drops  carried  aloft  by  the  powerful 
current  of  air  are  frozen  into  clear  hailstones  of  the  ordinary  size, 
which,  upon  being  thrown  outward  beyond  the  influence  of  the  as- 
cending current,  fall  to  the  earth.  The  snowy  nucleus  of  other  hail- 
stones is  supposed  to  be  formed  as  a  minute  snowball  above  the 
region  of  rain,  and,  in  descending,  to  be  several  times  drawn  into 
the  ascending  current  and  repeatedly  carried  aloft  before  it  reaches 
the  earth,  each  time  receiving  a  layer  of  ice  in  the  region  of  rain, 
and  of  snow  in  the  higher  regions. 

Dew  differs  from  fog  or  cloud  in  being  little  globules 
of  water  condensed  from  the  atmospheric  vapor,  not  in  the 
air,  but  upon  cool,  solid  bodies  which  have  chilled  the  ad- 
jacent air  below  its  point  of  saturation. 

If,  in  a  warm  room,  a  tumbler  be  filled  with  ice  water,  the  outside 
of  the  glass  will  in  a  few  minutes  be  clouded  over  with  myriads  of 
tiny  water  globules.  These  are  in  every  way  analogous  to  dew,  and 
are  caused  by  the  chilling  of  the  air  adjacent  to  the  cold  glass,  and 
the  consequent  condensation  of  its  vapor  upon  the  outside  of  the 


MOISTURE   OF   THE   ATMOSPHERE.  75 

\t         V 

glass.  The  flitting  cloud  seen  on  a  polished  knife  blade  when 
breathed  upon,  is  similarly  caused  by  its  momentarily  chilling  the 
warm  breath  and  condensing  part  of  its  vapor. 

Natural  Formation  of  Dew. — At  night  the  eai 
radiates  more  heat  than  it  receives,  and  becomes  cooler. 
Clouds  absorb  and  reflect  back  most  of  this  heat,  and  so 
maintain  the  temperature  of  the  earth  throughout  the 
night;  but  if  Jte  sky  be  clear,  the  temperature  of  surface 
objects  may  fall  below  the  point  of  saturation  of  the  ad- 
jacent air.  When  this  happens,  the  excess  of  vapor  in 
the  thin  layer  of  air  next  to  the  objects  condenses  into  tiny 
water  globules,  which  unite  into  dew-drops  upon  such  cold 
surfaces  as  leaves  and  grass  blades.  As  dew  is  thus 
formed  as  soon  as  the  temperature  of  the  air  sinks  below 
its  point  of  saturation,  the  temperature  of  the  point  of 
saturation  is  frequently  called  the  dew  point. 

Hoar-frost. — If  the  dew  point  of  the  air  be  below  the 
freezing  point,  the  excess  of  vapor  will  be  precipitated  as 
fine  spikelets  of  ice,  which  constitute  hoar-frost.  Hoar- 
frost is  not  frozen  dew,  but  a  sublimate \  i.  e.y  vapor  pre- 
cipitated in  a  solid  form. 

Both  dew  and  hoar-frost  are  precipitated  most  copiously  upon 
such  objects  as  cool  fastest,  and  thus  become  the  coldest.  Grass, 
trees,  and  herbage  generally,  though  no  better  radiators  than  the 
soil  or  rocks,  cool  faster,  because,  being  isolated,  they  lose  heat  by 
radiation  faster  than  they  receive  it  from  below  by  conduction. 

Distribution  and  Amount  of  Precipitation. — The 
total  amount  of  water  precipitated  upon  the  earth  in  all 
forms  is  for  convenience  called  rain-fall.  The  amount  of 
rain-fall  received  by  the  earth  as  a  whole  each  year  is  about 
equal  to  the  amount  of  water  evaporated,  but  the  amount 
of  rain-fall  received  by  the  land  is  greater  than  the  evap- 
oration from  its  surface,  while  evaporation  is  greater  than 
rain-fall  on  the  sea  surface.  On  the  average,  the  land 
loses  by  evaporation  about  three  fourths  of    its   rain-fall, 


MOISTURE   OF    THE    ATMOSPHERE.  J  J 

while  about  one  fourth  drains  into  the  ocean,  thus  main- 
taining its  level  against  the  excess  of  evaporation  from  its 
surface.  The  rain-fall  on  the  land  amounts  to  about  30,000 
cubic  miles  of  water  annually,  —  enough  to  cover  the 
whole  land  surface  to  a  uniform  depth  of  33  inches.  But 
all  parts  of  the  land  do  not  receive  equal  quantities  of 
rain-fall.  The  accompanying  chart  indicates  the  distribu- 
tion of  mean  annual  rain-fall  over  the  land. 

The  reasons  for  the  peculiar  distribution  will  appear  in  the  chapter 
on  Climate,  but  it  may  be  stated  here  (1)  that  the  vapor  taken  up  by 
the  winds  from  the  ocean  is  the  ultimate  source  of  rain-fall  on  the 
land ;  (2)  that  all  sea  winds  reach  the  land  nearly  saturated  with 
vapor ;  (3)  that  such  winds  in  warm  latitudes  contain  much  more 
vapor  than  in  cold  latitudes  ;  and  (4)  that  the  vapor  in  any  wind  is 
condensed  into  rain-fall  only  by  the  cooling  of  the  air.  This  is  usu- 
ally achieved  either  by  the  rising  of  the  air  or  by  its  entrance  into 
colder  latitudes. 

Evaporation,  like  rain-fall,  varies  in  different  localities  and  in  the 
same  locality  at  different  times.  It  is  most  active  where  the  wind  is 
strong  and  the  air  is  relatively  warm  and  dry,  but  it  may  cease  alto- 
gether if  the  amount  of  vapor  in  the  air  is  great.  It  is  always 
active  in  any  region  when  the  wind  is  blowing  from  a  colder  region, 
or  when  the  air  is  sinking,  for  in  both  cases  the  air  is  becoming 
warmer  and  its  relative  humidity  is  consequently  decreasing. 


CHAPTER  V. 

MOVEMENTS   OF  THE   ATMOSPHERE. 

The  wind  goeth  toward  the  south,  and  turneth  about  unto  the  north  ;  it  whirl- 
eth  about  continually,  and  the  wind  returneth  again  according  to  his  circuits.— 
Ecclesiastes  i :  6. 

Wind. —  Sensible  movements  of  air  are  called  wind. 
Winds  are  caused  by  the  force  of  gravity,  —  the  same  force 
that  causes  the  flow  of  rivers.  Gravity,  however,  could 
not  produce  movement  in  the  river  had  not  moisture,  in 
the  form  of  vapor,  first  been  raised  to  a  higher  level. 
This  is  accomplished  by  the  energy  of  the  sun's  heat. 
The  sun's  heat  also  enables  gravity  to  produce  winds. 

Cause. — The  sun  makes  some  parts  of  the  earth's  sur- 
face warmer  than  others.  The  warmer  part  heats  the  air  in 
contact  with  it.    This  air  consequently  expands.    The  expan- 

.^ -_ ^- sion  may  not  affect  the 

^-^^^^^^^^^^^ highest  layers  of  the  at- 

'^^~~—  ^^/^  mosphere,  but  it  pushes 

~\-—        _   i     ^  the  lower  layers  up  into  a 

"A '    convexity  over  the  warm- 

ly'.„„„„^rth'8p  ***#<**,  ?  er  regions,  as  in  Fig.  32. 
'^^^^^^^^m^i?}^^^^^^^  Gravity  now  causes  the 
Flg'32,  movement  indicated  by 

the  arrows,  for,  as  the  result  of  expansion  below  A,  the 
air  above  is  compressed  and  rendered  denser  than  that 
over  B.  As  part  of  the  air  over  A  thus  moves  away,  the 
weight  or  pressure  on  D  tends  to  decrease,  and  to  increase 
over  C ;  but  to  equalize  these  pressures,  the  lower  air 
moves  as  a  wind  toward  D.  The  warm,  expanded  air  is 
(78) 


MOVEMENTS    OF   THE  ATMOSPHERE. 


79 


lighter  than  the  surrounding  cool  air,  and  is  forced  by  it 
to  rise,  thus  forming  an  ascending  current  over  the  warm 

region,  while  over  the      . 

surrounding  cooler  re- 
gion the  air  is  gradually 
settling    downward    as 
the   bottom  air  moves 
from    under    it.      The 
general  movements  in- 
dicated  by  the  arrows  Fie-  33- 
in  Fig.  33,  result.     As  the  air  thus  rises  over  but  one  lo- 
cality,   while   it   sinks    down    in    the    entire    surrounding 
region,   it  must  ascend  faster  than  it  descends. 

The  movements  continue  as  long  as  the  central  air  is  warmer 
than  that  surrounding,  for  so  long  the  densities  are  unequal,  and 
gravity  produces  movement.  Thus,  every  wind  that  blows  on  the 
earth's  surface  has  its  counterpart,  blowing  in  a  different  direction, 
at  some  distance  above  that  surface.  The  lower  wind  blows  toward 
a  region  of  low  pressure,  where  the  air  is  rare  and  rising,  and  from 

a  region  of  high  pressure, 
where  the  air  is  dense  and 
sinking. 

The  Rotation  of  the 
Earth  appears  to  deflect 
all  winds  from  a  straight 
course.  The  deflection 
is  to  the  right  in  the 
northern,  but  to  the  left 
in  the  southern  hemi- 
sphere. 

Suppose  figure  34  to  rep- 
resent the  northern  hemi- 
sphere, and  a  wind,  shown 


Fig.  34- 


by  the  arrow  at  a,  to  be  blowing  poleward  along  the  meridian  A. 
While  the  wind  is  advancing  to  b,  c,  and  d,  the  rotation  of  the  earth 
carries  meridian  A  forward,  say  to  the  positions  of  B,  C,  and  D  re- 


8o 


PHYSICAL    GEOGRAPHY. 


spectively.  The  change  in  the  direction  of  the  meridian,  conse- 
quent upon  its  change  of  position,  causes  the  direction  of  the  wind, 
which  was  northward  at  a,  to  become  successively  more  and  more 
easterly  at  b,  c,  and  d;  and  as  we  are  apt  to  regard  the  direction  of 
the  meridian  as  fixed,  an  apparent  deflection  of  the  wind  to  the  right 
is  the  result.  The  same  cause  produces  a  gradual  northward  deflec- 
tion of  a  wind  blowing  due  west  over  meridian  E,  as  rotation  carries 
the  meridian  of  the  wind  successively  to  positions  F,  G,  H.  A  wind 
blowing  south  on  meridian  /  appears  to  turn  westward,  as  rotation 
carries  its  meridian  to  positions  J,  K,  L  ;  while  a  wind  blowing  due 
east  on  M  appears  to  turn  southward  as  its  meridian  advances  to 
N,  O,  P.  Thus,  a  wind  blowing  in  any  direction  in  the  northern 
hemisphere  appears  to  turn  to  the  right  from  its  original  course  as  it 
advances.  In  the  southern  hemisphere,  the  apparent  deflection  is 
to  the  left,  because  when  we  change  our  point  of  observation  from 
the  north  to  the  south  pole,  the  direction  of  the  earth's  rotation  ap- 
pears to  be  reversed.  Figure  34  accurately  illustrates  the  cause  of 
the  apparent  deflection,  but  exaggerates  its  amount.  Really  there  is 
no  deflection  of  winds  at  the  equator;  but 
on  leaving  the  equator,  the  amount  of  the 
deflection  increases  first  rapidly,  and 
then  very  slowly,  and  is  greatest  near  the 
poles. 

The  Effect  of  this  Deflective 
Tendency  is  to  prevent  the  winds 
from  moving  directly  toward  a  warm 
region.  Starting  directly  toward  it, 
the  winds  are  deflected  as  they  ad- 
vance, and  so  approach  the  warm 
region  obliquely ;  hence,  when  winds 
from  all  directions  blow  toward  some 
central  area,  the  deflective  influence 
causes  them  to  form  a  spiral  whirl 
around  the  central  area.  The  direc- 
tion of  the  whirl  is  obviously  to  the 
left  of  an  observer  at  its  center  in 
the  northern  hemisphere,  but  to  the  right  in  the  southern 
hemisphere. 


IN   SOUTHERN   HEMISPHERE 


Fig.  35- 


MOVEMENTS    OF    THE    ATMOSPHERE. 


81 


In  approaching  a  central  point,  the  winds  move  as  if  confined 
in  constantly  narrowing  paths,  and  hence  blow  with  increasing 
violence  as  they  advance.  This  is  because  the  air  that  crosses  the 
broader  portion  of  the  path,  near  the  margin  of  the  whirl,  must  cross 
the  narrow  portion,  near  its  center,  in  equal  times,  in  order  to  make 
room  for  the  following  air. 

Pressure  in  the  Whirl. — When  water  has  a  rapid 
rotary  motion,  as  in  an  eddy,  its  surface  is  observed  to  be 
depressed  near  the  center  and  elevated  near  the  circumfer- 
ence of  the  whirl.  This  is  caused  by  the  centrifugal  force 
developed  by  its  rotary  motion.  The  same  force  is  devel- 
oped by  the  rotary  motion  of  air,  and  caus.es  a  decrease 
of  atmospheric  pressure  in  the  center  of  a  whirl,  from 
which  the  pressure  increases  gradually  to  its  circumfer- 
ence. 

The  lowest  layer  of  air,  being  greatly  impeded  by  friction  on  the 
earth's  surface,  does  not  rotate  so  fast  as  the  next  higher  layer,  and 
each  layer,  being  less  dense,  offers  less  frictional  resistance  to  the 
stratum  above;  hence,  the  upper  strata  develop  great  centrifugal 
force  and  a  large  central  area  of  depression,  while  the  lower  strata 
develop  less  centrifugal  force  and  a  small  area  of  depression.  The 
lower  winds,  pushed  by  the  greater  pressure  behind,  flow  spirally 
toward  the  central  area,  where  they  slowly  ascend ;  they  move  fast- 
est near  the  center,  and  as  they  flow  spirally  outward  aloft,  their 
velocity  decreases. 

Three  Classes  of  Winds. — Since  winds  are  caused 
by  inequalities  in  the  weight  or  density  of  the  atmos- 
phere in  adjacent  regions,  and  since  these  inequalities  of 
weight  are  caused  primarily  by  differences  of  temperature, 
winds  may  be  divided  into  three  classes  according  to  the 
permanence  of  their  exciting  cause:  (i)  As  equatorial  re- 
gions are  always  warmer  than  polar  regions,  there  must  be 
winds  constantly  blowing  toward  the  equator  in  the  lower 
atmosphere,  and  from  the  equator  in  the  higher  atmos- 
phere. These  may  be  called  Constant  winds.  (2)  As  the 
land  and  water  surfaces  have  different   temperatures,   the 


82  PHYSICAL    GEOGRAPHY. 

land  being  generally  warmer  in  summer,  and  the  water  in 
winter,  there  must  be  winds  blowing,  in  the  lower  atmos- 
phere, toward  the  land  during  one  part  of  the  year,  and 
from  the  land  during  another  part  of  the  year.  These  may 
be  called  Periodic  winds.  (3)  The  whole  of  a  land  or  water 
surface  is  seldom  equally  heated;  some  places  are  hotter 
than  others,  owing  to  local  or  temporary  causes.  There 
are,  therefore,  temporary  winds  blowing  in  the  lower  at- 
mosphere toward  these  warmer  places  from  all  surround- 
ing regions.     These  may  be  called  Occasional  winds. 

Constant  Winds. — The  high  temperature  near  the 
equator  creates  a  belt  of  expanded  and  rising  air,  toward 
which  surface  winds  blow  from  the  northern  and  southern 
hemispheres,  and  from  which  the  upper  winds  move  over 
either  hemisphere.  The  movements  cause  a  belt  of  low 
pressure  along  the  thermal  equator,  and  since  the  upper 
winds,  moving  from  the  equator,  are  advancing  from  all 
directions  toward  a  common  center  (the  pole),  they  gradu- 
ally form  an  immense  whirl,  which  in  turn  causes  an  area 
of  low  pressure  near  either  pole. 

Tropical  Belts  of  High  Pressure. — Between  the 
equatorial  belt  of  low  pressure,  caused  primarily  by  heat, 
and  the  polar  low  pressure,  caused  directly  by  the  whirl 
of  the  winds,  there  must  be,  in  either  hemisphere,  a  belt 
of  relatively  high  pressure.  The  mean  position  of  this 
belt  is  in  the  neighborhood  of  that  parallel  which  divides 
the  surface  of  either  polar  hemisphere  into  two  equal 
parts — the  parallel  of  300  N.  or  S.  latitude.  It  has  been 
seen  that  the  lower  air  is  pushed  out  in  all  directions  from 
under  an  area  of  high  pressure  toward  areas  of  lower  pres- 
sure. Consequently,  surface  winds  issue  from  the  tropical 
belts  of  high  pressure  toward  the  equatorial  low  pressure 
belt  on  one  side,  and  toward  the  areas  of  polar  low  pressure 
on  the  other  side. 


MOVEMENTS    OF   THE    ATMOSPHERE.  83 

The  Trade  Winds. — On  the  equatorial  side  the  winds 
advance  readily,  being  urged  forward  by  the  high  pressure 
into  regions  where  the  air  is  warmer  and  lighter.  Conse- 
quently, they  blow  with  great  steadiness  throughout  the 
year.  They  are  gradually  deflected  to  the  westward  by 
the  earth's  rotation,  and,  since  winds  are  named  by  the 
direction  from  which  they  blow,  they  become  north-east 
winds  on  the  northern  and  south-east  winds  on  the  south- 
ern side  of  the  thermal  equator.  Their  uniformity  in  force 
and  direction  won  for  them  the  name  trade  winds,  because, 
like  trade,  they  follow  a  fixed  or  trodden  path.  Their 
mean  velocity  is  about  6}i  miles  an  hour. 

The  Antitrade  Winds. — The  surface  winds  which 
issue  from  the  polar  sides  of  the  tropical  belts  of  high 
pressure,  are  urged  forward  by  that  pressure  into  regions 
where  the  air  cools  and  becomes  heavier.  This  frequently 
impedes  the  advance  of  the  air,  and  consequently  the 
winds  are  not  so  constant  as  the  trade  winds.  By  the 
earth's  rotation  they  become  south-west  winds  in  the 
northern,  and  north-west  winds  in  the  southern  hemisphere ; 
and  since  these  directions  are  opposite  to  those  of  the  trade 
winds,  these  winds  are  called  the  antitrade  winds.  Since 
these  winds  form  part  of  the  great  polar  whirl,  their  ve- 
locity increases  as  they  approach  the  center  of  the  whirl 
(page  81).  Their  mean  velocity  on  the  Atlantic  in  500 
latitude  is  about  30  miles  an  hour. 

Belts  of  Calms. — In  the  belt  of  low  pressure  near  the 
thermal  equator,  where  the  motion  of  the  air  is  upward, 
and  in  the  tropical  belts  of  high  pressure  where  the  motion 
of  the  air  is  downward,  the  movement  is  largely  insensi- 
ble, and  calms  or  light,  variable  winds  are  the  result. 
These  calm  belts  travel  northward  and  southward  as  the 
sun  becomes  vertical  over  different  latitudes  in  different 
seasons  of  the  year.     In  the  Pacific,  the  equatorial  calms 


84  PHYSICAL    GEOGRAPHY. 

extend  south  of  the  equator  in  January,  but  lie  entirely 
north  of  it  in  July;  while  over  the  greater  part  of  the 
Atlantic  they  are  north  of  the  equator  during  the  entire 
year— about  2°  north  in  January,  and  about  io°  north  in 
July.  The  trade  and  antitrade  winds  and  the  belts  of 
calms  are  better  defined  on  the  ocean  than  on  the  conti- 
nents, because  the  sea  surface  has  a  more  uniform  temper- 
ature, and  because  that  surface  is  smoother  than  the  land 
and  offers  less  frictional  resistance  to  the  winds. 

The  constant  winds  are  more  plainly  marked  in  the  southern 
hemisphere  than  in  the  northern  hemisphere,  because  there  is  but 
little  land  in  the  south  temperate  zone  to  become  in  turn  hotter  and 
colder  than  the  surrounding  ocean  as  the  seasons  change,  and  thus 
to  modify  the  direction  of  the  winds.  Therefore,  the  Wind  Charts 
of  the  Southern  Hemisphere,  on  the  opposite  page,  are  given  first. 
On  these  is  shown  the  direction  of  the  prevailing  winds  in  summer 
(January)  and  in  winter  (July).  Isobars,  or  lines  drawn  through 
places  where  the  atmospheric  pressure  is  the  same,  are  also  shown, 
the  isobars  denoting  the  mean  or  a  high  pressure  (30  inches  or  over) 
being  drawn  in  red,  while  isobars  of  low  pressure  (less  than  30 
inches)  are  drawn  in  blue.  It  is  seen  that  a  belt  of  high  pressure 
lies  over  each  of  the  oceans  in  about  300  latitude  in  January,  while 
in  July  this  belt  of  high  pressure  almost  encircles  the  hemisphere 
in  this  latitude.  The  winds  blow  obliquely  out  from  these  regions 
of  high  pressure,  forming  the  trade  winds  on  the  equatorial  side, 
and  the  antitrade  winds  on  the  polar  side. 

Periodic  Winds. — In  the  neighborhood  of  the  conti- 
nents the  direction  of  the  trade  and  antitrade  winds  is  con- 
stantly undergoing  a  gradual  change,  owing  to  the  seasonal 
variation  in  the  relative  temperature  of  the  land  and  water ; 
hence,  in  such  regions  the  constant  winds  become  periodic 
winds.  There  are  two  kinds  of  periodic  winds:  seasonal 
winds  and  diurnal  winds. 

Monsoons. — Most  of  the  land  on  the  globe  lies  in  the 
north  temperate  zone.  In  these  latitudes,  it  has  been  seen, 
the  land  is  warmer  than  the  adjacent  ocean  in  summer,  but 
colder  than  the  ocean  in  winter.     In  summer,  therefore. 


High   Pressure  ■  jp.r ■  "VS^^-g 

Mean        "       -a* — 


86  PHYSICAL   GEOGRAPHY. 

the  air  over  the  land  is  the  more  expanded,  and  forms  a 
region  of  relatively  light  air,  toward  which  the  surface 
winds  blow  from  the  surrounding  oceans,  and  in  which 
they  escape  by  ascending  to  the  upper  atmosphere. 

In  winter,  on  the  contrary,  the  warmer  oceanic  air  is  the 
more  expanded,  and  the  colder  land  air  is  relatively  dry 
and  dense.  The  surface  winds  at  this  season  consequently 
blow  outward  in  all  directions  toward  the  ocean  from  the 
land  region  of  greatest  density  where  the  air  is  sinking. 
These  winds,  blowing  toward  the  land  in  summer  and  from 
the  land  in  winter,  are  called  monsoons,  from  an  Arabic 
word  meaning  season. 

Since  uniformity  of  temperature  is  more  disturbed  by  a  large 
land  surface  than  by  a  small  one,  the  monsoons  of  continents  are 
stronger  and  steadier  than  those  of  islands,  and  those  of  large  con- 
tinents than  those  of  small  continents.  Since  vapor  obstructs  the 
passage  of  heat  rays,  and  since  the  amount  of  vapor  in  the  atmos- 
phere decreases  upward  very  rapidly,  the  surface  of  high  land  is 
heated  in  summer,  and  is  cooled  (by  radiation)  in  winter  much  more 
readily  than  low  land  with  its  moister  atmosphere.  Hence,  a  conti- 
nent composed  of  highlands  will  have  much  stronger  monsoons 
than  a  low  continent.  The  great  influence  of  the  extensive  land 
masses  in  the  north  temperate  zone  in  modifying  the  direction  of  the 
winds  in  their  neighborhood  at  different  seasons  is  well  shown  on 
the  Wind  Charts  of  the  Northern  Hemisphere.  Thus,  in  southern 
and  eastern  Asia,  and  over  the  eastern  and  western  portions  of 
North  America,  the  direction  of  the  winds  in  January  is  almost  oppo- 
site to  that  prevailing  in  July. 

The  Monsoons  of  Asia  and  Australia. — Owing  to 
the  peculiar  position  of  Asia  with  relation  to  the  Indian 
Ocean,  to  its  vast  extent,  and  to  the  occurrence  in  that 
grand  division  of  the  most  extensive  region  of  very  high 
land  on  the  globe  (the  plateau  of  Thibet),  the  monsoons  of 
the  northern  Indian  Ocean  and  the  Malay  Archipelago  are 
particularly  well  marked. 

In  summer,  the  heat  upon  the  Asiatic  highlands  is  greater,  and  the 
air  is  less  dense  than  that  on  the  equatorial  Indian  Ocean,  and  the 


High   Pressure 

Mean         " 

Low         "     2UI* 


^  Isobars  every  ffi  inch. 
Arrows  fly  with 
the  winds. 


88  PHYSICAL    GEOGRAPHY. 

southern  trade  winds  of  that  ocean  sweep  north  of  the  equator. 
Here,  influenced  by  the  earth's  rotation,  they  veer  to  the  right  and 
reach  the  coast  of  Arabia  and  India  as  the  south-west  monsoon. 
This  monsoon  blows  steadily  from  May  to  October.  Southerly  and 
easterly  monsoon  winds  prevail  at  this  season  on  the  south-east  and 
east  coasts  of  Asia,  northerly  winds  in  the  northern  part  of  the 
grand  division,  and  north-westerly  and  westerly  winds  blow  over 
Europe. 

In  winter,  all  this  is  changed.  At  that  season  Asia  is  colder  than 
the  adjacent  oceans,  and  the  air  over  it  becomes  very  dry  and  dense. 
The  winds  blowing  from  this  region  of  very  dense  air  from  October 
to  May  are  influenced  by  the  earth's  rotation,  and  become  the 
steady  north-east  monsoon  of  the  north  Indian  Ocean,  the  north-west 
monsoon  of  the  east  coasts  of  Asia,  southerly  winds  in  Siberia,  and 
easterly  or  south-easterly  winds  in  eastern  Europe. 

The  Monsoons  of  the  other  Grand  Divisions  are 
similar  but  not  so  pronounced  as  those  of  Asia,  owing  to 
their  smaller  size  and  lower  surface.  The  North  Amer- 
ican regions  of  low  pressure  in  summer  and  high  pressure 
in  winter  are  quite  perceptible,  however,  and  the  latter,  in 
connection  with  the  high  pressure  existing  over  Asia  at 
that  season,  has  a  marked  influence  upon  the  winter  winds 
of  the  intervening  oceans. 

Effect  on  Winds  of  North  Atlantic  and  Pacific 
oceans. — Since  the  eastern  and  western  continents  nearly 
touch  at  Bering  Strait,  the  very  dense  air  lying  over  the 
continents  in  winter,  and  that  composing  the  belt  of  per- 
manent high  pressure  over  the  tropical  oceans,  quite  sur- 
round the  northern  parts  of  the  Atlantic  and  Pacific 
respectively,  over  each  of  which  the  air  is  rare  and  the 
pressure  low.  The  surface  winds  flowing  from  all  sides 
into  these  regions  of  lighter  air,  are  deflected  by  the 
earth's  rotation  into  a  great  whirl  over  each  of  the  oceans. 
The  center  of  these  whirls  is  near  the  parallel  of  6o°,  in 
which  latitude  the  difference  between  continental  and 
oceanic  temperatures  is  greatest  (page  87,  January). 


MOVEMENTS    OF    THE    ATMOSPHERE.  89 

Diurnal  Winds  of  Coasts. — Near  the  coast  the  land 
air  has  nearly  the  same  mean  temperature  as  the  adjacent 
sea  air;  but  since  it  rests  on  a  land  surface,  the  air  be- 
comes slightly  warmer  during  the  day  and  slightly  cooler 
at  night  than  the  sea  air.  A  sea  breeze  consequently 
springs  up  during  the  forenoon  and  blows  inland  until 
night-fall,  when,  after  a  short  calm,  a  land  breeze  begins 
to  blow  toward  the  sea,  and  continues  until  morning.  On 
tropical  coasts,  these  breezes  occur  regularly  throughout 
the  year;  in  higher  latitudes  they  are  not  noticeable  in 
winter,  the  land  air  being  so  chilled  by  radiation  during 
the  long  nights  of  that  season  that  it  fails  to  attain  the 
temperature  of  the  sea  air  during  the  short  days. 

Diurnal  Winds  of  Mountain  Valleys. — Since  the 
earth's  surface  is  quickly  heated  by  the  sun's  rays  by  day, 
and  quickly  cooled  by  radiation  at  night,  and  since  this 
relatively  hot  or  cold  surface  largely  governs  the  tempera- 
ture of  the  air  resting  upon  it,  it  follows  that  the  air  rest- 
ing on  highlands  may  become  hotter  by  day  than  the  air 
at  the  same  altitude  over  adjacent  valleys  or  lowlands. 
When  thus  heated  and  expanded,  the  highland  air  flows 
off  above  and  increases  the  pressure  in  the  valleys.  The 
increase  of  pressure  drives  surface  winds  up  the  valleys  by 
day.  At  night  the  highland  ground  and  the  air  resting 
on  it  may  become  cooler  than  the  air  at  the  same  altitude 
over  the  lowland.  It  contracts  in  cooling,  and  upper  cur- 
rents begin  to  move  toward  it,  thus  tending  to  increase 
the  pressure  on  the  highland,  and  drive  surface  winds 
down  the  valleys  by  night. 


P  G. 


CHAPTER  VI. 

MOVEMENTS    OF   THE    ATMOSPHERE — Continued. 

Occasional  Winds  include  all  winds  which  are  usually 
called  storms.  They  also  include  many  winds  which  are 
similar  to  storm  winds,  but  are  not  so  violent.  They  may 
occur  in  any  latitude,  but  are  very  much  more  frequent 
between  the  parallels  of  400  and  yo°  than  they  are  in 
other  latitudes. 

Whirling  Motion. — Since  occasional  winds  are  directly 
caused  by  the  difference  in  density  between  a  compara- 
tively small  central  region  where  the  density  is  relatively 
slight,  and  the  surrounding  regions,  where  the  density  is 
relatively  great,  the  air  of  the  surrounding  regions,  in 
moving  toward  the  central  region,  gradually  acquires  a 
whirling  or  rotary  motion,  which  is  characteristic  of  all 
occasional  winds. 

The  whirl  may  begin  as  a  comparatively  small  affair,  sometimes  but 
a  few  miles  in  diameter ;  but  by  the  decrease  of  pressure  in  the  central 
area,  owing  to  the  centrifugal  force  of  the  rotating  winds,  the  diame- 
ter of  the  whirl  may  increase  to  2,000  miles.  The  force  of  the  winds 
which  constitute  the  whirl  gradually  increases  from  the  margin 
toward  the  center  of  the  whirl.  Thus,  it  may  be  a  gentle  breeze 
near  the  margin,  and  be  blowing  with  the  hurricane  force  of  100 
miles  or  more  an  hour  near  the  center. 

Progressive  Movement. — In  addition  to  the  whirling 
motion,  occasional  winds  have  also  a  progressive  move- 
ment ;  that  is,  the  center  of  the  whirl,  instead  of  remain- 
ing stationary,  moves  from  place  to  place.  Neither  the 
direction  nor  the  speed  of  this  motion  is  regular,  but  it  is 
(90) 


92  PHYSICAL    GEOGRAPHY. 

generally  in  nearly  the  direction  of  the  prevailing  wind 
in  which  the  whirl  occurs.  The  general  movement  is 
westward  and  away  from  the  equator  in  the  torrid  zone, 
but  eastward  and  away  from  the  equator  in  the  temperate 
zones.  The  average  speed  is  from  8  to  14  miles  an  hour 
in  the  torrid  zone,  but  from  17  to  28  miles  an  hour  in 
higher  latitudes. 

These  Moving  Whirls  constitute  occasional  winds. 
The  direction  of  the  wind  at  any  place  depends  upon  the 
position  of  the  center  of  the  whirl 
with  relation  to  the  place  at  the  time. 
Thus,  suppose  A  and  B  (Fig.  36) 
to  be  two  places  500  or  more  miles 
apart,  but  both  lying  in  the  anti- 
trade wind  region  of  the  northern 
hemisphere.  Let  the  long  arrow, 
represented  as  flying  north  of  east, 
indicate  the  general  direction  of  the 
antitrade  wind,  and  the  direction  in  which  a  great  whirl, 
represented  by  the  small  arrows,  is  progressing.  It  will 
be  seen  that,  as  the  whirl  passes,  the  wind  at  A  is  first 
south-east,  then  east,  and  finally  north-east;  while  at  B 
the  wind  is  first  south-west,  then  west,  and  finally  north- 
west. Occasional  winds  may  be  broadly  divided  into 
three  classes:  (1)  dust  whirlwinds,  (2)  cyclones,  and  (3) 
tornadoes. 

Dust  Whirlwinds  are  essentially  the  draining  away, 
upward,  of  a  thin  layer  of  calm,  dry  air,  which  has  be- 
come excessively  heated  by  contact  with  the  sun-warmed 
earth.  As  sunshine  is  required  to  heat  the  earth,  these 
winds  occur  only  in  the  day-time.  Once  formed,  they 
continue  until  the  layer  of  heated  air  has  drained  away  or 
been  cooled  by  contact  with  the  cooling  earth  after  sun- 
set.    As  vegetation  affords  a  protection  against  the  sun's 


MOVEMENTS   OF    THE    ATMOSPHERE.  93 

heat,  dust  whirlwinds  are  most  frequent  over  deserts  or 
hot  turnpike  roads.  Although  strong  enough  to  carry 
along  dust,  sand,  straw,  and  leaves,  these  whirlwinds  sel- 
dom attain  a  disastrous  force  because  of  their  short  dura- 
tion and  consequent  small  diameter,  and  also  because  of 
the  friction  with  the  earth's  surface  of  the  thin  stratum  of 
air  in  motion. 

In  the  intensely  hot  and  sandy  deserts  of  tropical  regions,  as  well 
as  in  Arizona  and  other  parts  of  the  West,  these  whirlwinds  attain 
their  greatest  development.  In  Africa  and  Arabia  they  are  known 
as  simooms,  and  are  dreaded  not  only  for  the  heat  of  the  wind,  but 
for  the  immense  clouds  of  sand  with  which  they  fill  the  air. 

Cyclones  differ  essentially  from  dust  whirlwinds  in  be- 
ing composed  of  moist  instead  of  dry  air.  Several  im- 
portant peculiarities  result  from  this  difference.  Moist  air 
is  heated  directly  by  the  sun's  rays  in  the  day-time,  and 
its  cooling  is  retarded  by  the  radiation  from  the  earth  at 
night ;  hence,  if  the  air  is  calm,  heat  may  accumulate, 
and  a  much  thicker  layer  of  air  may  become  excessively 
warm  before  it  begins  to  drain  upward  than  in  the  case 
of  the  dust  whirlwind.  When  movement  begins,  where 
the  air  is  most  expanded  and  moist,  the  air  cools  as  it 
ascends,  and  a  part  of  its  vapor  condenses  into  clouds; 
hence,  rain  generally  accompanies  a  cyclone.  The  con- 
densation of  the  vapor  liberates  latent  heat,  which  pre- 
vents the  ascending  moist  air  from  cooling  rapidly.  From 
all  these  causes  it  results  that  a  cyclone  is  not  a  mere  day- 
time whirl  like  the  dust  whirlwind.  It  generally  continues 
for  several  days,  and  may  grow  from  the  effects  of  cen- 
trifugal force,  so  as  to  involve  in  the  whirl  all  the  air 
within   1,000  miles  or  more  of  the  center. 

A  cyclone  generally  continues  and  increases  in  size 
until  the  air,  ascending  in  the  central  column,  is  no  longer 
able  to  flow  off  above,   beyond  the  outer  margin  of  the 


94  PHYSICAL    GEOGRAPHY. 

cyclone.  Then,  as  more  air  flows  in  below  than  flows  out 
above,  the  rotation  slowly  ceases,  the  pressure  rises  at  the 
center,  and  the  cyclone  dies  away.  The  progression  of  a 
cyclone  frequently  carries  it  half-way  round  the  earth  be- 
fore it  dies  away,  as  shown  by  the  chart  on  page  91. 

In  tropical  regions,  cyclones  are  most  violent  because  the  heat 
and  moisture  are  greatest.  They  are  called  hurricanes  in  the  trop- 
ical Atlantic,  and  typhoons  in  the  Pacific  and  Indian  oceans.  Since 
cyclones  require  calm  air  for  their  formation,  they  originate  in  the 
torrid  zone  in  general  only  in  the  equatorial  calms;  but  as  their  de- 
velopment depends  upon  the  deflective  influence  of  the  earth's 
rotation,  which  decreases  to  nothing  at  the  equator,  hurricanes  are 
most  frequent  in  summer,  when  the  equatorial  calms  are  most  dis- 
tant from  the  equator,  and  they  rarely  occur  in  the  south  Atlantic 
because  these  calms  never  extend  far  south  of  the  equator.  In  the 
south  Indian  and  Pacific  oceans,  also,  typhoons  are  most  common 
in  summer  (February).  In  the  north  Indian  Ocean,  typhoons  are 
most  frequent  at  the  change  of  the  monsoon,  at  which  season  light, 
variable  winds  and  calms  prevail.  Since  these  storms  require  some 
time  to  grow,  and  as  they  are  traveling  westward  and  away  from  the 
equator  while  developing,  it  is  the  western  part  of  the  tropical 
oceans  in  which  cyclones  are  most  frequent  and  most  violent.  The 
cyclones  of  temperate  zones  are  less  violent  than  tropical  cyclones, 
because  the  atmospheric  heat  and  vapor  are  less  in  these  latitudes ; 
but  they  are  much  more  numerous,  because  the  deflective  influence 
of  the  earth's  rotation  is  so  much  greater  that  local  areas  of  low 
pressure  are  more  apt  to  grow  into  great  cyclones.  These  areas  of 
low  pressure  occur  in  great  numbers  in  temperate  latitudes  as  a 
result  of  the  cooling  of  the  antitrade  winds  as  they  advance  into 
higher  latitudes,  and  the  consequent  formation  of  local  calms  and 
areas  of  relatively  low  pressure.  Because  of  the  great  numbers  of 
these  that  develop  into  cyclones  or  storms,  the  regions  between  400 
and  700  latitude  are  the  great  storm  zones  of  the  world.  This  is 
particularly  true  north  of  the  equator,  where  the  irregularities  of 
the  greater  land  surface,  by  directing  the  air  upward,  promote  the 
formation  of  cyclones. 

Rain  Area  in  a  Cyclone. — In  the  temperate  zones 
most  of  the  rain  accompanying  a  cyclone  falls  east  of  its 
center,  for  it  is  on  this  side  that  the  winds  of  the  whirl 


MOVEMENTS    OF    THE  ATMOSPHERE.  95 

move  into  regions  where  they  become  colder,  and  in  con- 
sequence of  this  much  of  the  vapor  is  condensed.  The 
greater  amount  of  latent  heat  thus  liberated  on  the  eastern 
side  of  a  cyclone  is  one  of  the  prime  causes  of  its  pro- 
gressive eastward  movement  in  the  temperate  zones. 

Anticyclones. — Many  cyclones  exist  simultaneously  in 
the  temperate  regions.  They  follow  each  other  in  rapid 
succession.  One  may  overtake  another,  and  they  may 
coalesce  into  but  one  larger  cyclone ;  or,  owing  to  local 
peculiarities  of  heat  and  moisture,  a  large  cyclone  may 
divide  into  two,  which  diverge  and 
pursue  slightly  different  courses.  The        1  (  *    ^ 

region  between  the  margins  of  ad-      1,      \   ( t  f^        * 
jacent  cyclones  is  of  course  a  region  ^J^  jjjj^^N  \ 

of  relatively   high   pressure,  and  is       ^__   ~^/)\\\ 
called  an  anticyclone,  because  it  dif-         ^  *  \    \ 

fers  in  almost  every  respect  from  a  /    /     J     ' 

cyclone.  Thus,  (a)  being  a  region  of  in  northern  hemisphere 
relatively  high  pressure,  the  surface  IN  southern  hemisphere 
winds  blow  in  all  directions  out  from  ^     \  \      I 

it  (Fig.   37)  instead  of  into  it ;    (b)  ^      \    t  /  / 

these   winds   are    deflected    by   the  <<~>+4!!**~-*" 

earth's   rotation  into  outward  mov-  /''/'(   \^T 

ing  spirals,   the  whirl   being  to  the  \    V    ^~~~" 

right  of  an  observer  at  the  center,  j       \        X 

in  the  northern  hemisphere,   and  to 

,        .     .  L  Fig.  37.— Anticyclones. 

the  left  in  the  southern,  and  conse- 
quently in  a  contrary  direction  to  the  movement  in  a 
cyclone;  and  (c)  as  the  air  descends  in  regions  of  rela- 
tively high  pressure  it  becomes  warmer,  and  therefore  its 
vapor  does  not  condense ;  hence,  anticyclonic  winds  do 
not  usually  bring  cloudy  or  rainy  weather.  Traveling  to 
regions  of  greater  differences  of  pressure,  these  winds 
move  faster  as  they  advance. 


96 


PHYSICAL    GEOGRAPHY. 


Tornadoes. — A  tornado  is  a  whirl  of  small  diameter, 
but  great  depth  and  velocity,  which  forms  a  short  distance 
above  the  earth's  surface,  and  into  which  the  surface  air 
is  "sucked  up"  with  excessive  violence.  It  is  believed 
that  tornadoes  constitute  a  small  secondary  whirl  within 
some  gently  moving  cyclone.  They  have  been  known  to 
form  at  all  hours  and  in  all  seasons,  but  they  occur  most 
frequently  during  sultry  afternoons  of  summer.     They  are 


Fig.  38.— A  Tornado. 


supposed  to  be  directly  caused  by  a  warm,  moist  current 
of  air  at  some  distance  above  the  earth's  surface,  but 
underneath  a  higher  current  of  cold,  dry  air.  The  moist- 
ure in  the  warm  layer  accumulates  heat  directly  from  the 
sun's  rays  and  by  the  radiation  from  the  earth,  and  be- 
comes excessively  hot  in  comparison  with  the  air  above. 
The  secondary  whirl,  or  tornado,  forms  about  some  point 
where  the  thick  layer  of  hot  air  begins  to  escape  upward, 
and  rotates  in  the  same  direction  as  the  cyclone  in  which 
it   is   formed.     As  the  moist   air  ascends,    expands,    and 


MOVEMENTS    OF    THE    ATMOSPHERE.  97 

cools,  the  vapor  condenses  and  forms  the  gyrating,  funnel- 
shaped  cloud,  hanging  small  end  downward,  which  serves 
as  a  warning  of  the  tornado's  approach.  The  funnel  is 
formed  some  distance  above  the  earth's  surface,  in  the  air; 
and  as  friction  is  there  very  slight,  the  winds  of  the  whirl 
attain  enormous  speed,  and  develop  great  centrifugal  force, 
which  causes  a  decided  decrease  of  pressure  in  the  funnel. 
The  surface  air  being  thus  suddenly  relieved  of  a  great 
portion  of  the  weight  of  the  air  above,  expands,  often  with 
explosive  violence,  and  rushes  with  great  rapidity  up  the 
funnel.  Violent  surface  winds  rush  in  from  all  sides  to 
take  its  place  and  follow  it  upward.  These  surface  winds 
constitute  the  destructive  blast  of  the  tornado. 

The  force  of  the  tornado  blast  is  terrific  ;  it  blows  down  the 
strongest  houses  and  largest  trees,  and  carries  such  heavy  objects  as 
carts,  iron  chains,  beams,  and  even  men  and  women  whirling  aloft 
in  the  gyrating  funnel.  To  produce  such  results,  a  wind  velocity  of 
over  200  miles  an  hour  is  thought  to  be  necessary.  The  tornado 
winds,  however,  seldom  attain  destructive  violence  over  a  track  ex- 
ceeding one  fourth  mile  wide.  Like  the  cyclone,  a  tornado  has  a 
progressive  motion,  usually  in  the  direction  of  the  prevailing  winds 
of  the  region  where  it  occurs.  In  the  United  States,  tornadoes 
usually  travel  north-east  at  a  speed  of  about  30  miles  an  hour.  The 
tornado  continues  until  the  layer  of  hot  air  has  drained  away ;  this 
usually  takes  about  an  hour ;  hence,  the  track  of  a  tornado  is  usu- 
ally about  thirty  miles  long. 

Thunder-storms  with  rain  and  hail  usually  accom- 
pany tornadoes.  The  vapor  of  the  ascending  and  expand- 
ing air  in  the  funnel  is  condensed  into  water,  which, 
carried  upward  by  the  powerful  updraught,  is  converted 
into  hail.  The  friction  of  the  rapidly  ascending  air  and 
water  particles  possibly  generates  the  electricity  mani- 
fested in  the  thunder-storms. 

Cloud  Bursts. — The  ascent  of  air  in  a  tornado  may  be 
so  violent  as  to  prevent  the  fall  of  rain-drops,  and  thus 
cause   an   enormous   accumulation    of   water    in    the    air. 


O  on      » 

IXl  <  Ol-o 

|_  U  uQ  o 
_iu.z*-  <x> 

Qi.5E 

•4*        i*        ^°  S, 


MOVEMENTS    OF   THE    ATMOSPHERE.  99 

Upon  the  cessation  of  the  updraught,  the  water  may  fall 
in  continuous  streams.  This  is  called  a  cloud-burst.  Each 
of  these  streams  may  excavate  a  great  hole,  or  basin,  in 
the  ground,  and  on  steep  slopes  may  occasion  a  land-slide 
or  a  great  ravine,  and  wash  large  rocks  and  trees  bodily 
down  the  hillside.  Such  a  cloud  burst  occurred  at  Spring- 
field, Ohio,  in  May  of  1886,  and  occasioned  great  dam- 
age, inundating  dwellings,  and  washing  away  railroad 
embankments. 

Water-spouts  and  White  Squalls. — When  a  tornado 
occurs  at  sea,  and  its  funnel-shaped  cloud  descends  to  the 
surface  of  the  water,  the  violently  agitated  water  is  sucked 
up  for  a  short  distance  into  the  funnel;  but  at  a  height  of 
a  few  feet  it  breaks  into  spray,  which  is  carried  aloft  by 
the  whirling  winds,  thus  presenting  the  appearance  of  a 
solid,  whirling  column  of  water,  or  water-spout.  White 
squalls  are  simply  small,  fair  weather  tornadoes.  They  fre- 
quently cause  water-spouts,  and  may  be  quite  violent. 

Frequency  of  Tornadoes. — During  the  twelve  years 
prior  to  1883,  over  500  tornadoes  occurred  in  the  United 
States,  or  an  average  of  one  every  nine  days.  They  occur 
in  Kansas,  Illinois,  and  Missouri  more  frequently  than 
elsewhere  in  the  Union.  In  this  region  the  warm  south- 
erly surface  winds  of  summer,  underrunning  the  colder 
westerly  winds  in  the  upper  atmosphere  which  have 
crossed  over  the  Rocky  Mountains,  afford  conditions  pecu- 
liarly favorable  for  tornado  formation.  The  accompany- 
ing chart  shows  the  relative  tornado  frequency  in  different 
parts  of  the  United  States.  The  deeper  shading  shows 
regions  where  tornadoes  are  more  frequent. 


CHAPTER  VII. 

LUMINOUS  PHENOMENA  OF  THE  ATMOSPHERE. 

By  what  way  is  the  light  parted  ?—] ob  xxxviii  :  24. 

Apparent  Displacement  of  Heavenly  Bodies  by 
Refraction. — The  sun,  moon,  and  other  heavenly  bodies 
are  visible  when  they  are  really  below  the  horizon,  owing 
to  the  refraction  (page  26)  of  their  rays  as  they  penetrate 
the  increasingly  denser  atmosphere  in  approaching  the 
earth's  surface.  In  the  torrid  and  temperate  zones  the  sun 
thus  appears  to  rise  in  the  morning  earlier  and  set  in  the 
evening  later  than  it  really  does  by  from  2  to  27  minutes, 

... ^  according  to  the  latitude 

a      x\  Co\        of  the  observer  and  the 

season  of  the  year. 

If  AB  represent  the  hori- 
zon line  of  an  observer  at 
A,  a  ray  from  the  center  of 
the  sun  or  moon,  C,  entering 
the  atmosphere  at  E,  is  refracted  into  a  curve  as  it  traverses  succes- 
sively denser  layers  of  air.  The  observer  at  A  sees  the  sun  in  the 
direction  in  which  the  ray  is  traveling  at  the  instant  it  enters  his  eye, 
or  at  D.  The  amount  of  this  displacement  decreases  from  about 
the  apparent  width  of  the  sun,  at  the  horizon,  to  nothing  at  the 
zenith,  where  the  rays  fall  with  no  obliquity.  This  explains  the  oval 
shape  sometimes  observed  in  the  sun  or  moon  when  near  the  hori- 
zon; for  the  lower  edge,  whose  rays  strike  the  atmosphere  more 
obliquely,  is  displaced  more  than  the  upper  side. 

The  Sun  and   Moon  appear  larger  near  the   Hori- 
zon than  when  higher  in  the  sky.     They  are,  of  course, 
(100) 


LUMINOUS    PHENOMENA. 


IOI 


not  really  any  larger  or  nearer  at  such  times,  jan.d  the.  ap- 
pearance has  nothing  to  do  with  refracfi^V  but -arises 
simply  from  a  common  error  of  judgment  on  the  part  of 
the  observer. 

All  objects  appear  smaller  as  their  distance  increases,  and  our 
only  means  of  judging  the  size  of  a  distant  object  whose  dimensions 
are  unknown,  is  by  comparing  it  with  the  apparent  size  of  some 
equally  distant  but  familiar  object — as  a  man,  a  tree,  a  house,  etc. 
When  low  in  the  sky,  the  sun  or  moon  is  in  a  position  where  it  may 
be  directly  compared  with  familiar  objects  on  the  distant  horizon, 
and  its  great  relative  size  impresses  itself  upon  us  and  makes  it  seem 
actually  larger  than  when  seen  higher  in  the  heavens  with  no 
standard  of  comparison  near  it.  For  the  same  reason  we  are  apt  to 
think  that  a  full  grown  man  at  the  top  of  a  steeple  200  feet  high  is 
only  a  boy.  No  one  makes  such  a  mistake  with  regard  to  a  man  at 
the  same  distance  when  he  is  surrounded  by  familiar  objects  on  the 
earth's  surface  which  serve  as  standards  for  comparison. 

Twilight. — After  the  sun  has  disappeared  below  the 
horizon,  the  earth  is  not  immediately  plunged  into  dark- 
ness :  objects  remain  visible  by  the  light  reflected  from  the 
higher  parts  of  the  atmosphere  which  is  still  traversed  by 
sunbeams.  This  is  called  twilight  (half  light).  The  same 
phenomenon  occurs  before  sunrise,  and  is  called  dawn. 

Even  when  the  sun  is  in  the  zenith  of  a  cloudless  sky,  as  much 
as  one  fifth  of  the  light  we  receive  is  that  which  is  reflected  to  us 
from  other  quarters  of  the  sky  than  that  through  which  the  beams 
penetrate  directly  to  us.  When  the  sun  is  just  above  the  horizon, 
more  than  two  thirds  of  our 
light  is  that  which  is  re- 
flected from  the  sky;  and 
when  it  is  invisible  below 
the  horizon,  all  our  light  is 
so  reflected  to  us.  Suppose 
the  ray  SE  (Fig.  40)  is  the 
last  from  the  setting  sun  which  strikes  A.  A  is  illuminated  by  this 
direct  ray,  and  by  rays  reflected  from  every  point  of  the  sky  from  H 
to  F  which  is  still  traversed  by  the  sun's  beams.  The  sun's  rays  can 
not  reach  A  when  rotation  has  carried  it  to  C,  and  lifted  its  horizon 


FULL     LIGHT 


IG2  PHYSICAL    GEOGRAPHY. 

to  G:  but  Cis  not  dark  because  reflected  light  from  every  point  of 
the  *ky  between  F  end  G  reaches  and  illuminates  it  with  twilight. 
When  the  horizon  is  lifted  to  F,  however,  by  the  earth's  rotation  to 
D,  neither  direct  nor  reflected  light  from  the  sun  reaches  that  part 
of  the  earth's  surface,  and  darkness  prevails. 

"The  Sun  drawing  up  Water." — When  a  ray  of 
light  is  admitted  into  a  dark  room,  its  path  becomes  visi- 
ble by  the  light  reflected  from  the  air  particles  and  float- 
ing dust  motes  (page  27).  The  same  phenomenon  on  a 
grand  scale  is  sometimes  seen  in  the  open  air  when  the  sun- 
beams, breaking  through  rifts  in  the  clouds,  are  rendered 
visible  in  the  clouds'  shadow  by  the  light  reflected  to  the 
eye  from  the  strongly  illuminated  dust  and  air  particles  in 
their  path.  This  phenomenon  is  frequently  but  errone- 
ously supposed  to  be  "the  sun  drawing  up  water." 

Mirage. — Adjacent  layers  of  air  near  the  earth's  sur- 
face have  sometimes,  owing  to  differences  in  temperature 
or  humidity,  widely  different  densities.  The  refraction 
and  total  reflection  of  light  rays  in  traversing  such  layers 
often  give  rise  to  distorted,  displaced,  or  inverted  images 
of  the  objects  from  which  they  proceed.  This  phenom- 
enon is  called  mirage.  The  suitable  atmospheric  condi- 
tions may  occur  in  any  region,  but  are  probably  most 
-^^_^  frequent  over  hot  deserts. 
^ftKS^  Looming  and  Fata  Morgana 
/\\  are  but  peculiar  instances 
t^^~  -— ->^z~-^^     of  mira^e- 

"-  -  '  """■"-.T!7"'^7~"'"^|>3  Suppose  the  heated  ground  has 

*"*■•  c     ji  warmed  the  lower  layers  of  the 

^^&«*        atmosphere,  A,  B,  C  (Fig.  41),  to 
-*'  a  much  higher  temperature,  and 

thereby  made  it  much  rarer  than 
the  air  above.  The  rays  indicated  by  the  dotted  line  reveal  the 
tree  to  the  observer  at  F  in  its  proper  position,  but  the  ray  striking 
the  layer  of  rare  air  at  E  is  refracted  more  and  more  as  it  enters 
rarer  layers,  until  it  strikes  a  layer  so  obliquely  as  to  be  totally  re- 


Fig.  41. 


LUMINOUS    PHENOMENA. 


IO3 


Fig.  42- 


fleeted  at  D.  The  observer  sees  an  inverted  tree  in  the  direction  G 
at  which  this  ray  enters  his  eye,  and  the  impression  conveyed  is 
that  the  real  tree  is  standing  on  the  bank  of  a  lake,  in  which  its 
inverted  reflection  is  seen.  The  cold  surface  of  the  sea  may  some- 
times so  chill  and  render  relatively  dense  the  lower  layers  of  the  at- 
mosphere, that  rays  passing  from 

a  vessel  at  A  (Fig.  42),  completely  ^S^d 

hidden  from  the  observer  at  Cby  ,.--''' 

the  rotundity  of  the  earth,  are  re- 
fracted downward  by  the  rarer 
layers  of  the  higher  atmosphere, 
and  totally  reflected  at  B,  thus 
producing  an  image  of  the  con- 
cealed vessel  in  the  clouds  at  D,  above  its  true  position,  and  in  the 
direction  which  the  ray  entered  the  observer's  eye.  The  image  may 
sometimes  be  erect,  sometimes  inverted,  and  may  be  greatly  en- 
larged. An  image  of  a  vessel  30  miles  distant  from  the  observer 
has  thus  been  seen,  and  the  image  of  the  French  coast,  which  is 
usually  invisible,  has  thus  been  lifted  into  the  view  of  those  on  the 
opposite  side  of  the  English  Channel.  Lateral  displacement  occurs 
when  the  layers  of  air  of  different  density  are  vertical,  or  more  or 
less  inclined  to  the  horizontal. 

Color  of  the  Atmosphere. — In  small  masses,  air  has 
no  appreciable  color.  In  large  masses  its  color  varies 
with  its  position  in  relation  to  the  sun.  If  the  sunlight 
passes  directly  through  the  air  to  the  eye,  we  see  the  air 
by  transmitted  light,  and  it  is  reddish  if  the  sun  is  near  the 
horizon,  but  yellowish  if  the  sun  is  high  in  the  heavens. 
If  the  air  we  observe  is  not  directly  between  the  eye  and 
the  sun,  we  see  it  by  the  sunlight  which  it  reflects  to  the 
eye,  and  it  appears  azure,  or  bluish.  It  is  the  color  of  the 
atmosphere,  thus  seen,  which  makes  the  sky  and  distant 
hills  or  mountains  appear  blue. 

It  has  been  seen  (page  28),  that  a  colorless  ray  of  sunlight, 
passing  through  a  prism,  is  refracted  and  separated  into  a  num- 
ber of  colored  rays.  The  only  difference  in  these  colored  rays 
is  thought  to  be  the  width  of  the  ether  waves,  or  vibrations,  which 
are  supposed  to  constitute  all  light;  the  blue  is  conceived  to  be 
caused  by  short,  quick  waves,  and  the  red  by  longer  and  slower 


104  PHYSICAL    GEOGRAPHY 

waves,  while  the  original  colorless  ray  is  supposed  to  be  composed 
of  waves  of  all  lengths,  unassorted.  Now,  the  atmosphere  is  known 
to  contain  myriads  of  floating  dust  motes  and  other  particles,  which 
vary  in  size  from  an  inconceivable  smallness  to  the  size  where  they 
can  no  longer  float,  but  fall  through,  the  air  to  the  earth's  surface. 
If  an  ordinary  water  wave  encounter  a  small  chip  or  other  floating 
object,  it  simply  passes  under  the  object  and  continues  its  course. 
If,  however,  it  encounters  a  much  larger  object,  the  wave  is  unable 
to  lift  it,  and,  striking  against  it,  breaks  and  rebounds,  that  is,  is  re- 
flected from  it.  The  floating  dust  motes  have  much  the  same  effect 
on  the  light  waves  passing  through  the  atmosphere.  The  shorter  blue 
waves  are  broken  and  reflected  not  only  by  the  larger  motes,  but  by 
those  which  are  too  small  to  reflect  the  longer  yellow  and  still  longer 
red  waves.  Thus,  when  a  ray  of  sunlight  passes  through  the  atmos- 
phere, more  of  its  blue  waves  than  of  its  yellow  and  red  waves  are 
reflected  to  the  eye,  and  hence  objects  seen  by  such  reflected  sun- 
light, in  which  the  short  blue  vibrations  predominate,  appear  more 
or  less  bluish.  When,  however,  a  ray  of  sunlight  passes  directly 
through  the  atmosphere  to  the  eye,  more  of  its  blue  waves  have  been 
reflected  into  other  directions,  and  the  reddish  and  yellowish  waves 
are  in  excess  in  the  transmitted  light.  When  the  sun  is  near  the 
horizon,  its  rays  pass  through  great  distances  of  the  lower  atmos- 
phere, which  contains  the  largest  motes,  and  the  yellow  as  well  as 
the  blue  waves  are  sifted  out  by  this  process  of  selective  absorption 
of  the  atmosphere  (page  28),  leaving  the  transmitted  light  reddish. 
The  remarkably  long  and  brilliantly  red  sunsets  and  twilights  of 
the  fall  and  winter  of  1883-4  are  thought  to  have  been  caused  by 
the  ordinary  action  of  this  selective  absorption.  They  were  remark- 
able, however,  because  the  amount  of  dust  in  the  higher  atmosphere 
was  unusually  large  at  that  time,  great  quantities  having  been  grad- 
ually diffused  over  the  entire  globe  from  the  terrific  volcanic  erup- 
tion of  August,  1883 — at  Krakatoa,  in  the  Strait  of  Sunda,  between 
Sumatra  and  Java  (page  285). 

Color  of  Clouds  and  Snow. — Masses  of  cloud  and 
snow  are  nearly  opaque,  although  composed  of  minute 
particles  of  transparent  water  or  ice ;  for  while  most  of  the 
light  falling  upon  each  particle  is  transmitted  through  it, 
and  but  little  reflected  from  it,  still  there  are  so  many  par- 
ticles, and  hence  so  many  minute  reflections,  that  all  the 
light  is  reflected  away  from  the  eye  before  it  can  traverse 


LUMINOUS    PHENOMENA.  IO5 

the  mass  of  cloud  or  snow.  When  seen  by  directly  re- 
flected sunlight,  and  not  too  distant,  the  sensation  appro- 
priate to  colorless  opaque  bodies  is  excited,  and  the  cloud 
or  snow  appears  milk  white ;  but  clouds,  if  very  distant 
and  high  above  the  horizon,  appear  bluish,  while  distant 
snow  or  clouds  near  the  horizon  appear  yellow  or  red,  be- 
cause the  dense  atmosphere  reflects  the  blue  waves  of 
their  rays  away  from  the  eye. 

The  Rainbow  is  the  beautiful  arc,  containing  all  the 
colors  of  the  spectrum,  which  is  usually  seen  through  a 
shower  or  heavy  mist,  upon  which  the  sun  shines  from  a 
point  behind  the  observer.  It  is  caused  by  the  separation 
of  white  sunlight  into  its  prismatic  colors  by  refraction  in 
the  water  drops,  and  the  total  reflection  of  these  colored 
rays  back  to  the  eye  of  the  observer.  Each  color  and 
each  point  in  the  arc  is  the  instantaneous  reflection  from  a 
separate  drop.  The  exterior  edge  of  the  rainbow  is  red, 
and  the  interior  edge  is  blue  or  violet.  Sometimes  a 
double  arc  or  rainbow  is  seen,  in  which  case  the  outer  one 
is  wider  but  fainter  than  the  in- 
ner one,  and  the  order  of  its 
colors  is  reversed. 

When  a  beam  of  sunlight  enters  a 
drop  of  clear  water  at  a  certain  angle, 
it  is  totally  reflected  from  the  interior 
surface,  and  emerges  on  the  same 
side  of  the  drop  at  which  it  entered. 
In  addition  to  this,  while  the  beam  is 
traversing  the  drop,  refraction  sepa- 
rates it  into  its  colored  ravs,  of  which 

Fig.  43. 

only  the  red,  yellow,  and  blue  ones 

are  indicated  in  Fig.  43.  If  the  position  of  this  drop  is  such  that  the 
reflected  red  ray  enters  the  eye  of  the  observer,  the  other  colored 
rays  pass  above  the  eye  and  the  drop  appears  red.  But  at  the  same 
instant  a  similar  phenomenon  takes  place  in  other  drops  at  such 
distances  be^w  the  first  that  only  their  yellow  and  blue  rays  re- 
spectively enter  the  eye,  causing  these  drops  to  appear  respectively 


106  PHYSICAL    GEOGRAPHY. 

yellow  and  blue, — the  lower  drop  appearing  blue.  Between  these 
drops  are  others  which  reflect  their  appropriate  color,  and  the  whole 
series  of  drops  gives  the  appearance  of  a  continuous  party-colored 
band,   the  red  above  gradually  changing  through   yellow  to  blue 

below.  Each  drop  occupies  but  for  an 
instant  the  proper  position  for  its  re- 
flection to  enter  the  eye,  but  this  posi- 
tion is  so  soon  occupied  by  a  following 
drop  that  the  sensation  is  continuous. 
The  angle  between  the  sunbeams  and 
the  reflected  rays  is  always  nearly  420. 
Each  part  of  the  shower  reveals  pris- 
lg'  **"  matic  colors  at  a  point  where  lines 

drawn  to  the  sun  and  the  eye  inclose  an  angle  of  420.  Collectively, 
these  points  form  the  curve  of  the  rainbow.  The  second  exterior 
rainbow  sometimes  seen  is  caused  by  the  refraction  and  two  reflec- 
tions of  the  sunbeam  within  the  drop  (Fig.  44). 

Halos  and  Coronas  are  sometimes  seen  around  and 
at  some  distance  from  the  sun  or  moon.  The  halos  result 
from  the  refraction  of  light  in  ice  crystals  which  compose 
the  highest  cirrus  cloud,  and  are  more  or  less  distinctly 
colored.  Coronas  may  be  colorless,  and  are  caused  by  the 
diffraction  from  the  surface  of  haze-  or  cloud-globules.  If 
these  are  small,  the  diameter  of  the  corona  is  great,  and 
vice  versa;  hence,  when  a  ring  is  seen  closely  encircling 
the  moon,  the  globules  of  water  in  the  clouds  are  known 
to  be  large,   and  rain  may  be  expected. 

Atmospheric  Electricity. — The  atmosphere  is  always 
more  or  less  highly  charged  with  electricity.  This  is  prob- 
ably a  result  of  evaporation,  and  the  friction  of  air  and 
vapor  particles  with  each  other.  Millions  of  vapor  parti- 
cles condense  into  a  single  cloud-globule ;  hence,  however 
small  the  electric  charge  on  a  single  vapor  particle,  the 
accumulation  in  a  cloud  mass  might  be  enormous. 

Lightning. — When  an  electrified  cloud  approaches 
another  cloud  or  the  earth  sufficiently  close,  its  electricity 
and  the  opposite  kind  induced  on  the  neighboring  cloud  or 


LUMINOUS    PHENOMENA.  IO7 

the  earth,  rush  together  through  the  intervening  air,  pro- 
ducing the  great  electric  spark  called  lightning.  This  elec- 
tricity travels  with  the  enormous  velocity  of  radiant  light 
and  heat  (186,000  miles  a  second).  A  flash  of  lightning, 
therefore,  though  often  one  mile,  and  sometimes  more 
than  five  miles  in  length,  seems  instantaneous.  There  are 
at  least  three  varieties  of  lightning:  (1)  forked  lightning, 
(2)  sheet  or  heat  lightning,  and  (3)  ball  lightning. 

Forked  lightning  is  a  sharp,  zigzag  line  of  dazzling  white  light, 
marking  the  line  of  least  resistance  through  the  dense  lower  air  be- 
tween two  highly  charged  clouds,  or  a  cloud  and  the  earth.  Sheet 
lightning  is  the  most  common  form,  and  occurs  as  a  broad  sheet  of 
rather  pale,  diffused  light.  Frequently  it  is  not  accompanied  by 
audible  thunder.  Usually  it  is  distant  forked  lightning,  but  some- 
times is  a  weak  electrical  discharge  within  a  cloud  at  a  considerable 
height  where  the  air  is  rare.  Ball  lightning  is  a  very  rare  form.  A 
vivid  flash,  accompanied  by  a  violent  explosion,  seems  to  project  a 
brilliant  bomb  to  the  earth.  Upon  striking  the  earth,  the  bomb  may 
rebound  several  times  before  it  splits  up  and  disappears.  No  satis- 
factory explanation  has  been  given  of  this  singular  form  of  electrical 
discharge. 

Thunder  is  simply  the  crackle  of  the  lightning  spark. 
It  has  the  same  cause  (page  34)  as  the  much  feebler 
crackle  of  the  smaller  sparks  produced  artificially.  The 
passage  of  electricity  is  so  rapid  that  the  crackle  is  pro- 
duced at  practically  the  same  instant  throughout  the 
length  of  a  lightning  flash  several  miles  long.  But  as 
sound  requires  almost  five  seconds  to  travel  one  mile,  it 
arrives  from  successively  more  distant  points  at  sensibly 
later  periods  of  time,  and  thus  produces  the  continuous 
roar  or  roll  of  thunder.  The  sound  is  further  prolonged 
and  repeated  by  being  reflected,  or  echoed,  from  the  sur- 
faces of  clouds,  the  earth,  and  masses  of  air  of  unequal 
density.  Thunder  is  seldom  heard  over  a  greater  distance 
than  twelve  miles. 


108  PHYSICAL    GEOGRAPHY. 

St.  Elmo's  Fire. — Atmospheric  electricity  of  very  low 
intensity,  such  as  often  occurs  in  fair  weather,  is  frequently 
sufficient  to  induce  in  prominent,  sharp-pointed  objects,  a 
greater  amount  of  electricity  than  the  attenuated  object 
can  hold,  and  what  is  called  a  brush  discharge  takes  place, 
without  audible  noise,  but  frequently  with  a  feebly  lu- 
minous glow.  This  glow  is  often  seen  at  the  ends  of 
lightning  rods  or  of  the  masts  and  spars  of  vessels,  and  is 
called  by  the  sailors  St.   Elmo's  Fire. 

The  Aurora  Polaris,  or  Polar  Light,  is  a  singular  and 
beautiful  phenomenon  seen  in  the  sky,  most  frequently  in 
high  latitudes,  but  occasionally  in  all  parts  of  the  earth. 
It  consists  of  luminous  clouds,  arches,  or  rays.  The  rays 
frequently  shoot  up  and  down  in  diverging  lines  from  the 
horizon,  and  appear  to  converge  in  the  zenith.  The 
aurora  is  usually  a  pale,  greenish  yellow,  but  sometimes 
is  crimson,  violet,  or  steel  blue.  In  the  northern  hemi- 
sphere, the  phenomenon  is  most  common  in  a  narrow 
zone  surrounding,  but  at  some  distance  from,  the  magnetic 
pole  of  the  earth. 

This  zone  embraces  the  Faroe  Islands,  and  crosses  central  Labra- 
dor, Hudson  Bay,  and  Point  Barrow  in  northern  Alaska,  and  then 
skirts  the  north  coast  of  Asia.  North  of  this  zone  the  aurora  is  gen- 
erally seen  in  the  southern  sky,  but  south  of  the  zone  in  the  northern 
sky.  The  height  of  the  aurora  varies  greatly,  but  its  average  alti- 
tude seems  to  be  about  ioo  miles,  and  hence  in  a  region  where  the 
atmosphere  is  exceedingly  rare.  The  cause  of  the  aurora  is  un- 
known ;  it  is  certainly  connected  with  the  magnetism  of  the  earth, 
and  probably  results  from  the  discharge  of  atmospheric  electricity. 


PART   III.— THE  SEA. 


CHAPTER  VIII. 

DEPTH,    COMPOSITION,    AND    TEMPERATURE. 

Thy  way  is  in  the  sea,  and  thy  path  in  the  great  waters. — Psalm  lxxvii  :  19. 

The  Sea  is  a  continuous  body  of  water  which  partly 
envelopes  the  earth,  forming  nearly  three  fourths  (73)4%) 
of  its  surface. 

Oceans. — The  polar  circles,  the  continents,  and  the 
meridians  from  their  southern  points  are  taken  as  the 
boundaries  of  five  great  divisions  of  the  sea,  called  oceans, 
which  vary  greatly  in  shape  and  extent. 

The  Pacific  is  the  largest  ocean.  It  is  oval  in  shape. 
The  greatest  width  in  an  east  and  west  direction  lies  along 
the  equator,  and  is  about  10,000  miles.  Its  length,  from 
tiering  Strait  to  the  Antarctic  Circle,  is  9,000  miles.  It 
embraces  71,000,000  square  miles,  —  about  one  half  (49^) 
of  the  total  sea  area. 

The  Atlantic  Ocean  is  next  in  size.  It  is  a  long  and 
narrow  channel,  extending  9,000  miles  between  the  polar 
circles,  with  an  average  width  of  3,600  miles.  Its  area  is 
34,000,000  square  miles,  or  24%  of  the  sea  surface. 

The  Indian  Ocean  is  roughly  circular  in  shape,  having 
a  diameter  of  about  6,300  miles,  and  an  area  of  28,000,000 
square  miles,  or  20%  of  the  sea  surface. 

P.  G.-7.  (109) 


IIO  PHYSICAL    GEOGRAPHY. 

The  Antarctic  Region,  lying  within  the  Antarctic  Circle, 
is  circular,  with  a  diameter  of  3,300  miles,  and  an  area  of 
7,000,000  square  miles,  or  ^%%  of  the  sea  surface. 
About  4,700,000  square  miles  of  this  region  have  never 
been  explored.  The  unexplored  region  is  supposed  to 
contain  a  low  continent  or  large  island-group  completely 
covered  by  a  continuous  ice  cap  more  than  2,000  feet 
thick,  which  terminates  on  all  sides  in  a  perpendicular 
cliff  of  ice  about  200  feet  high  above  sea-level. 

The  Arctic  Ocean  is  really  a  great  gulf  of  the  Atlantic, 
extending  for  3,300  miles  from  Iceland  to  Bering  Strait. 
It  has  a  width  of  less  than  2,500  miles,  and  an  area  of 
4,000,000  square  miles,  or  2^^  of  the  sea  surface. 


THE  ttl.OHE     1                                                                                                                                                      ! 

PACIFIC 

ATI  ANTin                                                                                                                                       wamm—mmm 

INDIAN 

_^_ 

ANTARCTIC  REGIONS 

_ 

ARCTIC...    _ ... 

_.    ■ 

UNITED  STATES 

n 

FJg«  45-— Relative  Areas  of  the  Oceans. 

Continuity  of  the  Sea. — The  Pacific,  Atlantic,  and 
Indian  oceans  are  wide  and  open  at  the  south,  where, 
together  with  the  Antarctic,  they  form  a  continuous  and 
uninterrupted  sea  as  far  north  as  Cape  Horn,  a  point  cor- 
responding in  latitude  to  central  Labrador  and  Denmark 
in  the  northern  hemisphere.  From  this  sea  each  ocean  ex- 
tends northward  as  a  great  bay  or  channel.  At  the  tropic 
of  Cancer  the  Indian  Ocean  encounters  Asia  and  ends;  at 
the  Arctic  Circle  the  Pacific  practically  ends  at  the  shallow 
and  narrow  Bering  Strait,  leaving  the  Atlantic  alone  to 
make  a  broad  connection  with  the  Arctic  Ocean,  and  carry 
the  continuity  of  the  sea  into  the  frozen  regions  about  the 
north  pole. 


DEPTH  OF  THE   SEA.  I  1 1 

Atlantic  Coast. — The  Atlantic,  and  its  northern  ex- 
tension, the  Arctic  Ocean,  send  the  greatest  number  of 
deep  indentations  into  the  land.  In  general  these  inden- 
tations have  comparatively  narrow  mouths,  and  form  great 
inland  seas.  Thus,  to  the  Atlantic  and  Arctic  basin  be- 
long the  Gulf  of  Mexico,  Hudson  Bay,  the  Gulf  of  Obi, 
White,  Baltic,  Mediterranean,  and  Black  seas.  For  this 
reason  the  Atlantic,  although  it  has  but  one  half  the  area, 
has  a  longer  coast-line  than  the  Pacific.  The  Atlantic 
coast  is  55,000  miles  long;  that  of  the  Pacific  but  47,000. 

The  Pacific  Coast,  while  much  more  regular  than  that 
of  the  Atlantic,  possesses  the  greatest  number  of  border 
seas,  partially  separated  from  the  main  ocean  by  chains  of 
islands.  Such  are  Bering,  Okhotsk,  Japan,  Yellow,  and 
China  seas,  the  seas  of  the  Malay  Archipelago,  the  Coral 
Sea  east  of  Australia,  and  the  expanses  of  water  within 
the  numerous  islands  of  southern  Chile,  British  America, 
and  Alaska. 

The  Indian  Ocean  is  peculiar  in  the  number  and  size 
of  the  great  open-mouthed  indentations  in  its  coast-line, 
such  as  the  Gulf  of  Aden,  the  Arabian  Sea,  the  Bay  of 
Bengal,  Timor  Sea,  and  the  Great  Australian  Bight. 

Average  Depth. — The  average  depth  of  the  sea  is 
2,150  fathoms  (1  fathom  equals  6  feet)  or  2^  miles. 
That  of  the  Pacific  is  2,500  fathoms  or  about  3  miles; 
that  of  the  Atlantic  and  Indian  oceans  is  2,000  fathoms 
or  2)(  miles;  and  that  of  the  polar  oceans  is  probably 
less  than  1 ,  000  fathoms  or  about  1  mile.  While  these  are 
the  average  depths,  there  are  places  where  the  depth  is 
much  greater,  and  others  where  it  is  much  less.  The  blue 
shading  in  the  charts  indicates  the  portions  of  the  earth's 
surface  that  would  still  be  covered  with  water  were  the 
surface  of  the  sea  lowered  2,000  fathoms.  The  white  por- 
tion of  the  chart  indicates  the  region  that  would  thus  be 


ii2 


PHYSICAL    GEOGRAPHY. 


DEPTHS  OF  T   E  SEA 

PACIFIC 
OCEAN 


ess  than  looo  fathoms 
000-2000  fathoms 

\2ooo-3ooo  f'ms 
3000-4000 
Over 
4000 


converted  into  dry  land.  A  depression  of  the  sea  surface 
of  4,500  fathoms  or  about  5  miles,  would  be  required  to 
convert  the  whole  surface  of  the  earth  into  dry  land. 

The  dotted  line  in  the  white  portion  of  each  chart  indicates  the 
shore-line  of  the  sea  were  its  surface  lowered  only  1 ,000  fathoms ; 
the  darker  blue  tinting  indicates  regions  that  would  still  be  covered 
with  water  were  the  present  sea  surface  lowered  3,000  fathoms — 5% 
miles — while  the  small  areas  of  solid  blue  east  of  Japan  and  the  West 
Indies  indicate  the  deepest  depressions  of  the  earth  which  would 
remain  sea  were  the  present  surface  lowered  4,000  fathoms  or  \]/z 
miles. 

Depths  of  the  Sea  Compared  with  Heights  of  the 
Land. — The  greatest  depressions  of  the  earth  are  about 


DEPTH   OF   THE   SEA. 


»3 


Relative  extent  of  high 
land. — 


DEPTHS  OF  the  SEA 

ATLANTIC  and 
INDIAN 

OCEANS. 


as  far  below,  as  the  highest  mountains  are  above  the  sea 
surface;  but  the  area  of  deep  depressions  is  very  much 
greater  than  that  of  high  elevations.  Thus,  about  83%  of 
the  sea  area,  or  114  million  square  miles,  is  more  than 
1,000  fathoms  deep;  while  but  9%  of  the  land,  or  about 
52^  million  square  miles,  has  an  elevation  greater  than 
1,000  fathoms  (6,000  feet)  above  sea-level.  This  elevated 
region  of  the  land  is  indicated  on  the  chart  by  the  darkest 
red  tint.  It  is  estimated  that  it  would  require  all  the 
solid  portion  of  the  planet  down  to  a  depth  of  1,600 
fathoms  below  sea-level  to  fill  the  greater  depressions  up 
to  that  depth. 


114  PHYSICAL   GEOGRAPHY. 

Configuration  of  the  Sea  Bottom. — The  sea  bottom 
is  much  smoother  than  the  surface  of  the  land.  It  sinks 
more  or  less  rapidly  from  the  shores  of  the  continents  to 
its  average  depth,  and  continues  as  a  vast,  gently  undulat- 
ing plain  to  the  opposite  continent. 

Submarine  Plateaus. —  Occasionally  an  undulation 
may  rise  gradually  to  an  elevation  a  mile  or  two  above  the 
plain,  and  continue  for  a  greater  or  less  distance  as  a 
plateau,  at  a  depth  of  1,000  or  2,000  fathoms,  before 
again  sinking.  The  narrower  of  these  plateaus  or  ridges 
correspond  in  a  general  way  with  the  broad  plateaus  of  the 
land,  but  it  is  probable  that  the  sea  bottom  far  from  land, 
contains  no  narrow  and  rugged  irregularities  comparable 
with  mountain  chains. 

The  Atlantic. — A  submarine  plateau,  or  broad  ridge,  extends 
along  the  middle  of  the  Atlantic  throughout  its  length,  and  sepa- 
rates its  basin  into  an  eastern  and  a  western  depression.  The  average 
depth  on  this  plateau  is  1,500  fathoms,  while  the  two  depressions 
into  which  it  divides  the  basin  of  the  Atlantic  sink  to  mean  depths  of 
over  2,500  fathoms. 

The  Pacific  basin  is  more  intricate  than  that  of  the  Atlantic.  Sub- 
marine ridges  from  the  south  polar  regions  towards  Chile  and  the  isth- 
mus of  Panama,  and  towards  New  Guinea,  separate  four  depressions 
from  the  main  depression.  The  latter  is  imperfectly  separated,  by  a 
series  of  discontinuous  ridges  extending  southeastward  from  Japan, 
into  a  northern  and  a  southern  basin,  each  of  which  sinks  to  a  general 
depth  of  nearly  3,000  fathoms  with  considerable  areas  of  much  greater 
depth. 

The  Indian  Ocean  is  freer  from  submarine  plateaus  than  either  of 
the  other  oceans,  but  a  short  one  appears  east  of  Madagascar,  and 
another  extends  south  from  the  west  coast  of  India. 

Composition  of  Sea-water. — Unlike  rain-water,  or 
that  which  is  common  in  lakes  and  rivers,  the  water  of  the 
sea  is  so  salt  and  so  bitter  as  to  be  undrinkable.  If  100 
pounds  of  sea-water  be  placed  in  a  clean  vessel  and 
allowed  to  evaporate,   about  3^  pounds  of  solid  matter 


COMPOSITION    OF    SEA-WATER. 

will  remain  after  the  liquid  has  disappeared, 
matter,  dissolved  in  sea-water,  makes  it  heavi 
than  fresh  water,  and  gives  to  it  the  peculiar 

The  amount  of  solid  matter,  and  hence  the  weight  or  density  of 
the  surface  water,  varies  slightly  in  different  parts  of  the  open  sea, 
being  greatest  in  the  trade  wind  regions,  where  evaporation  is 
greater  than  the  rain-fall,  and  least  in  equatorial  regions  where  the 
rain-fall  is  in  excess,  and  in  the  polar  seas,  where  the  melting  ice 
supplies  a  great  amount  of  fresh  water.  In  partially  inclosed  seas 
or  bays  the  amount  of  solid  matter  in  solution  may  increase  to  about 
4$,  as  in  the  Mediterranean  and  Red  seas ;  or  decrease  to  iy2  %,  as 
in  New  York  Bay,  or  to  i^%,  as  in  the  Black  and  Baltic  seas,  ac- 
cording as  the  evaporation  and  the  salt  water  received  from  the 
ocean  is  greater  or  less  than  the  amount  of  fresh  water  received 
from  rain-fall  and  rivers.  The  relative  density  or  saltness  of  differ- 
ent parts  of  the  sea  is  shown  in  the  chart  on  page  138. 

The  Solid  Matter. — While  the  amount  of  solid  matter 
varies  slightly,  its  composition  remains  practically  the  same 
in  all  parts  of  the  sea.  Rather  more  than  three  fourths  of 
it  is  chloride  of  sodium,  or  common  salt.  The  sea  thus 
contains  in  solution  enough  common  salt  to  form  a  solid 
layer  126  feet  thick  over  the  entire  globe.  The  remaining 
portion  of  the  solid  matter  (other  than  salt)  gives  the  pe- 
culiar bitter  taste  to  sea-water,  and  consists  of  chloride  of 
magnesia,  Epsom  salts,  gypsum,  and  traces  of  nearly 
every  known  mineral,  minute  quantities  of  each  being 
dissolved  in  water  percolating  through  the  rocks  of  the 
land,   and  carried  eventually  by  the  rivers  to  the  sea. 

The  percentage  of  the  principal  solids  dissolved  in  sea-water  is 
as  follows  : 

Chloride  of  sodium  (common  salt)     .     .  '  .     .     .  77758% 

Chloride  of  magnesia 10.878 

Sulphate  of  magnesia  (Epsom  salts) 4-737 

Sulphate  of  lime  (gypsum) 3.600 

Sulphate  of  potassium 2.465 

Carbonate  of  lime  (limestone),  and  all  others      .  0.562 


Il6  PHYSICAL    GEOGRAPHY. 

Gaseous  Matter. — In  addition  to  its  solid  or  mineral 
ingredients,  sea-water  always  contains,  dissolved,  a  greater 
or  less  quantity  of  the  atmospheric  gases, —  oxygen,  nitro- 
gen, and  carbonic  acid.  Bubbles  of  air  composed  of  these 
gases  become  entangled  in  the  waves  of  the  sea  surface, 
and  the  gases  are  dissolved  and  gradually  diffused  to 
the  greatest  depths.  The  quantity  of  gas  so  dissolved 
amounts  to  from  2  %  to  3  %  of  the  volume  of  the  sea  (equal 
to  a  layer  of  air  230  feet  thick  surrounding  the  earth),  in 
the  proportion  of  about  *^  oxygen,  J^  carbonic  acid,  and 
y^,  nitrogen.  The  oxygen  is  a  little  more  abundant  in  the 
water  near  the  surface,  and  the  proportion  of  carbonic  acid 
increases  toward  the  bottom.  It  is  the  oxygen  thus  dis- 
solved in  sea-water  which  enables  submarine  animals  to 
live.     They  inhale  it  and  exhale  carbonic  acid. 

Cause  of  the  Saltness  of  the  Sea. — It  is  believed 
that  the  mineral  ingredients  of  sea-water  were  principally 
derived  from  the  mineral  gases  in  the  atmosphere,  when 
its  water  vapor  first  condensed  to  form  the  sea,  at  an  early 
period  of  the  planet's  history ;  and  hence  that  sea-water 
has  always  been  salty. 

At  that  time,  the  earth's  surface  was  much  hotter  than  it  is  now, 
and  great  quantities  of  the  minerals  which  exist  as  solids  at  the 
present  temperatures,  existed  then  as  gaseous  components  of  the  at- 
mosphere. Under  the  enormous  pressure  of  such  an  atmosphere, 
vapor  might  condense  into  clouds  and  rain  at  temperatures  now  re- 
quired to  melt  iron.  Hot  water  dissolves  a  much  greater  quantity 
of  most  minerals  than  cold,  and  such  hot  rain  falling  through  such  a 
mineral  laden  atmosphere  would  reach  the  earth  strongly  impreg- 
nated by  the  mineral  gases  through  which  it  had  passed. 

There  are  processes  now  at  work,  however,  which, 
by  continually  adding  small  quantities  of  similar  minerals, 
tend  to  gradually  increase  the  saltness  of  the  sea.  Sea- 
water,  in  evaporating,  leaves  all  its  impurities  behind. 
The  vapor  condenses  and  falls  as  nearly  pure  rain-water. 


TEMPERATURE    OF    THE    SEA.  II7 

Part  of  it  falls  on  the  land,  and  only  reaches  the  sea  again 
after  a  long  journey  in  some  stream  or  river.  During  this 
journey  it  dissolves  and  carries  away,  in  solution,  minute 
quantities  of  the  soils  and  rocks  with  which  it  comes  in 
contact.  River  water,  however  clear,  is  thus  never  pure. 
Upon  entering  the  sea,  it  adds  its  mite  to  the  quantity  of 
mineral  matter  in  solution. 

While  there  are  350  parts  of  mineral  matter  in  10,000  parts  of 
sea-water,  there  are  but  2  parts  of  mineral  matter  in  an  equal  quan- 
tity of  river  water ;  and  being  in  so  small  a  proportion,  it  does  not 
appreciably  affect  the  taste.  The  solution  in  river  water  depends 
upon  the  nature  of  the  rocks  encountered.  The  minerals  of  lime- 
stone, granite,  and  sandstone,  which  constitute  so  large  a  portion 
of  the  rocks  of  the  earth,  form  about  three  fourths,  and  common 
salt  but  a  small  part  of  the  mineral  matter  in  river  water,  while 
the  proportions  of  the  substances  are  just  reversed  in  sea-water* 
The  reason  for  this  is  explained  on  page  243. 

Temperature. — In  general,  the  surface  water  of  the 
sea  is  the  warmest.  Its  temperature  varies  from  about 
8o°  near  the  equator  to  about  300  near  the  poles.  The 
temperature  of  the  water  on  the  sea  bottom,  however,  is 
about  350  under  the  equator,  and  about  290  under  the 
polar  oceans.  Thus,  while  there  is  a  difference  of  500  be- 
tween the  surface  temperatures  of  the  polar  and  equa- 
torial oceans,  there  is  a  difference  of  only  6°  between  the 
temperatures  of  the  water  on  their  bottoms. 

Surface  Waters. — The  great  difference  between  the 
temperatures  of  polar  and  equatorial  surface  waters,  results 
from  the  different  heating  power  of  the  sun's  rays  when 
falling  almost  vertically,  as  near  the  equator,  and  very 
obliquely,  as  near  the  poles.  As  the  sun  is  vertical  over 
the  tropic  of  Cancer  in  June,  and  over  the  tropic  of 
Capricorn  in  December,  the  surface  waters  in  either  hemi- 
sphere are  alternately  warmer  and  colder,  according  to 
the  season  of  the  year.     This  seasonal  difference  of  tern- 


u8 


PHYSICAL    GEOGRAPHY. 


perature  is  very  slight  near  the  equator  and  near  the  poles, 
but  in  the  oceans  of  the  temperate  zones  it  amounts  to 
about  io°.  Thus,  in  the  latitude  of  New  York,  the  tem- 
perature of  the  sea  surface  is  between  500  and  6o°  in 
winter,  and  between  6o°  and  yo°  in  summer. 

Water  Beneath  the  Surface. — Since  water  is  a  very 
poor  conductor  of  heat,  the  direct  influence  of  the  solar 
rays  is  confined  to  a  comparatively  thin  layer  of  surface 
water.  From  the  surface  the  temperature  first  falls  rapidly 
as  the  depth  increases,  then  more  slowly,  and  then  with 
extreme  slowness,  either  to  the  bottom  or  to  a  certain 
depth,  whence  it  remains  nearly  uniform  to  the  bottom. 


80°N.  LATITUDE     60aN. 


P  A  C  I  F  .1  C 

Bering  Sea.    j/leutian  la.  f\  Hawaiian  is 


OCEAN 

/8.  a      .',  Friendly  Is. 


Fig.  46. 


Warm  and  Cold  Water  of  the  Sea. — Even  under 
the  equator,  a  temperature  of  400  is  always  reached  within 
a  depth  of  800  fathoms,  and  in  higher  latitudes  at  less 
depths.  Thus,  the  great  mass  of  sea-water  has  a  temper- 
ature below  400,  or  removed  but  a  few  degrees  from  its 
freezing  point.  The  portion  of  the  sea  which  has  a  tem- 
perature above  400  forms  a  comparatively  thin  layer  at  the 
surface  of  the  temperate  and  equatorial  oceans.  In  the 
sectional  diagrams,  this  layer  is  indicated  by  solid  black. 

Ice  of  the  Sea. — Salt  water  freezes  at  a  lower  temper- 
ature than  fresh  on  account  of  its  saltness.  The  surface 
water  of  the  sea  does  not  begin  to  solidify  into  ice  until 
its  temperature  falls  below  290  Fahrenheit. 


ICE    OF    THE   SEA.  119 

In  freezing,  salt  water  discards  most  of  its  salt,  and  expanding, 
becomes  comparatively  fresh  ice.  The  discarded  salt,  mixing  with 
the  water  immediately  beneath  the  ice,  makes  it  Salter,  and  there- 
fore it  does  not  freeze,  although  as  cold  as  the  surface.  It  also 
makes  it  heavier,  and  causes  it  to  sink  and  cool  the  deeper  water, 
which  is  thus  eventually  cooled  to  about  290  entirely  to  the  bottom. 

Ice-fields  and  Floes. — In  the  frigid  zones,  the  sur- 
face of  the  sea  is  annually  frozen  into  vast  fields  of  thick 
ice,  hundreds  of  miles  in  extent.  The  movements  of  the 
water  and  variations  in  its  temperature  cause  the  ice  to 
crack  into  immense  pieces,  or  floes,  the  pressure  of  which, 
one  against  the  other,  breaks  off  and  squeezes  up  huge 
fragments,  until  the  whole  surface  of  the  floe  becomes  so 
rough  and  uneven  that  traveling  over  it  is  almost  impossi- 
ble. The  portions  of  the  ice-field  near  the  shore  are  fre- 
quently covered  with  masses  of  rock  and  soil  loosened 
from  overhanging  cliffs  by  the  frosts  of  the  long  Arctic 
winter.  Thousands  of  tons  of  such  land  rubbish  are  thus 
annually  carried  to  sea  when  the  ice-floe  becomes  de- 
tached in  the  early  summer,  and  may  be  transported  hun- 
dreds of  miles  before  the  ice  melts  and  allows  its  load  to 
sink  to  the  bottom. 

Icebergs. — Very  different  from  the  comparatively  low 
ice-floe,  both  in  appearance  and  in  manner  of  formation, 
are  the  great  icebergs,  sometimes  200  or  300  feet  high, 
occasionally  seen  floating  in  the  Atlantic  as  far  south  as 
the  latitude  of  Washington,  and  frequently  observed 
stranded  in  the  shallow  waters  around  Newfoundland.  Un- 
like the  ice-floe,  bergs  are  not  frozen  sea-water,  but  are 
land  ice.  They  are  formed  of  the  snow,  which,  falling  to 
great  depths  on  polar  lands,  and  accumulating  in  the  con- 
tinued cold  to  still  greater  depths,  becomes  consolidated 
under  the  pressure  of  its  own  weight  into  solid  ice,  which 
covers  the  greater  part  of  the  land,  and  increases  in  thick- 
ness with  the  accumulation  on  its  surface,  until  its  weight 


(120) 


ICE    OF    THE    SEA.  121 

causes  it  to  move  gradually  downward  as  a.  glacier  over  the 
surface  of  the  land  into  the  sea.  When  it  has  advanced  to 
a  depth  of  water  greater  than  about  nine  tenths  of  the 
thickness  of  the  ice  sheet,  its  buoyancy  causes  great 
blocks  to  break  off  and  drift  away  as  icebergs. 

Observations  at  the  foot  of  the  Muir  glacter  lead  Professor  G.  Fred- 
erick  Wright  to  believe  that  most  icebergs  owe  their  detachment 
from  the  parent  glacier,  not  to  the  buoyancy  of  the  ice,  but  to  the 
fact  that  the  advance  of  the  glacier  is  faster  near  its  surface  than 
near  its  bed  (p.  233).  Fragments  of  the  surface  ice,  of  greater  or 
less  volume,  are  thus  pushed  off  over  the  submerged  foot  of  the  gla- 
cier, and  these  fragments  float  away  as  icebergs. 

In  Antarctic  seas,  the  icebergs  are  usually  about  175  feet  high,  and 
sometimes  3  miles  long,  with  flat  and  nearly  level  tops.  As  only 
about  one  tenth  of  the  mass  of  a  berg  protrudes  above  the  heavy 
sea-water,  such  icebergs  must  extend  to  a  depth  of  about  1,750  feet. 
As  the  iceberg  drifts  with  the  currents  into  warmer  latitudes,  it  be- 
comes very  irregular  in  shape  through  unequal  melting,  and  thus 
may  turn  completely  over  several  times,  and  reach  comparatively 
low  latitudes  before  its  great  mass  is  entirely  dissolved.  In  its  jour- 
ney over  the  land  and  sea  bottom  before  it  breaks  away  from  the 
parent  ice  sheet,  great  masses  of  stone  and  gravel,  becoming  em- 
bedded in  its  under  surface,  are  torn  from  their  places  and  distrib- 
uted over  hundreds  of  miles  of  sea  bottom  by  the  gradual  melting 
of  the  iceberg.  The  chart  on  page  138  shows  the  regions  in  which 
floating  ice  may  be  encountered. 


CHAPTER  IX. 

WAVES    AND    TIDES. 

Thou  rulest  the  raging  of  the  sea;  when  the  waves  thereof  arise,  thou  stillest 
them. — Psalm  lxxxix:  9. 

Movements  of  the  Sea. — The  water  of  the  sea  is  in 
constant  motion.  Its  movements  may  be  broadly  divided 
into  three  different  classes,  namely,  waves,  tides,  and  cur- 
rents. 

Waves. — The  ordinary  waves  of  the  sea  are  caused  di- 
rectly by  the  impact  or  friction  of  the  wind.  A  small 
local  agitation  of  the  surface  water  is  thus  produced,  which 
spreads  rapidly  in  all  directions  as  a  succession  of  undula- 
tions or  waves.  This  phenomenon  may  be  produced  by 
blowing  upon  or  along  the  surface  of  the  water  in  a  basin. 
Under  the  continued  action  of  the  wind,  these  waves  soon 
grow,  in  the  deep  and  open  sea,  into  great  billows.  Be- 
yond the  region  of  the  wind  which  caused  them,  the  bil- 
lows advance  with  gradually  diminishing  height,  and  may 
even  reach  the  shores  of  the  continents.  As  there  is 
never  a  time  when  the  winds  are  not  blowing  and  creating 
waves  in  some  parts  of  the  sea,  its  surface,  even  in  regions 
where  the  wind  is  not  blowing,  is  almost  always  heaving 
with  the  "  groundswell, "  or  the  diminishing  undulations 
of  waves  created  in  some  other  regions. 

Movement  of  the  Water  in  Waves. — While  the 
undulation  advances  rapidly  in  one  direction,  the  water 
itself  does  not  partake  of  this  continuous  progressive 
movement.     The  motion  of  the  water  is  indicated  by  that 


WAVES    AND   TIDES.  1 23 

of  a  floating  cork,  which  is  observed  to  rise  and  fall  as  the 
wave  passes,  but  otherwise  to  remain  in  nearly  the  same 
position. 

In  reality,  the  water  advances  while  on  the  upper  half  and  re- 
cedes while  in  the  lower  half  of  the  wave,  each  particle  moving  in  a 
nearly  circular  path  whose  diameter  equals  the  height  of  the  passing 
wave.  This  is  illustrated  in  Fig.  47.  A  cork  at  a,  on  the  front  slope 
of  an  advancing  wave,  A,  reaches  b  as  the  wave  reaches  X,  c  when 
the  wave  arrives  at  Y,  d  as  the  wave  reaches  Z,  and  a  again  when 
the  wave  has  advanced  its  length  to  B.  As  the  cork  advances  when 
above  and  recedes  when  below  the  medial  line  ef,  and  as  the  por- 
tion of  the  wave  ae  above  that  line  is  shorter  than  the  portion  af 
below,  the  advance  of  the  cork,  while  equal  in  amount,  is  more 


Direction  of  motion  of  wave     <*»- 


Fig.  47- 

rapid  than  its  recession.  The  enormous  energy  of  waves  is  due  to 
this  slight  but  exceedingly  rapid  (page  17)  advance  of  the  great 
body  of  water  while  in  the  crest  of  the  wave. 

Size  and  Speed  of  Waves. — The  size  of  waves 
(height  from  trough  to  crest,  and  length  from  crest  to  crest) 
in  the  open  ocean  depends  upon  the  force  and  continuance 
of  the  wind.  The  speed  increases  with  the  size.  The 
largest  wind  waves  are  about  50  feet  high  and  half  a  mile 
long,  and  travel  at  the  rate  of  80  miles  an  hour.  The 
ordinary  storm  waves  are  not  more  than  30  feet  high  and 
600  feet  long.     They  travel  about  37  miles  an  hour. 

Depth  of  Water  Affected  by  Waves. — In  the  waves 
of  the  open  sea,  all  motion  is  confined  to  a  comparatively 
thin  layer  of  surface  water.  At  a  depth  equal  to  the 
wave  length,  the  motion  is  less  than  -^o-th  part  of  that  at 
the  surface.     Thus,  the  largest  waves  of  the   sea,  which 


124  PHYSICAL    GEOGRAPHY. 

attain  a  height  of  50  feet,  cause  a  corresponding  move- 
ment of  but  one  inch  at  a  depth  of  half  a  mile  (440 
fathoms),  while  the  motion  of  smaller  waves  becomes  in- 
sensible long  before  a  depth  equal  to  their  length  is 
reached.  The  motion  arising  from  the  ordinary  waves  of 
the  sea  is  probably  quite  insensible  at  a  greater  depth  than 
100  feet  (17  fathoms). 

Breakers. — When  waves  from  the  deep  open  sea  reach 
water  so  shallow  that  perceptible  wave  motion  reaches  to 
the  bottom,  the  increased  friction  retards  the  bottom  of 
the  advancing  wave,  and  those  following  begin  gradually 
to  overtake  it.  The  waves  thus  become  shorter  and 
higher,  and  their  slopes,  especially  the  front  slope,  be- 
come steeper  as  the  water  becomes  shallower.  Finally, 
the  front  slope  of  the  wave  in  the  shallowest  water  be- 
comes so  steep  that  the  water  in  its  crest  falls  forward  in 
a  graceful  curve,  and  dashes  upon  the  beach,  amidst  foam 
and  spray,   as  a  breaker. 

Small  waves,  caused  directly  by  the  wind,  break  thus  where  the 
depth  of  water  below  the  trough  has  decreased  to  about  one  half 
the  height  of  the  wave.  The  larger  undulations  of  the  ground- 
swell,  however,  sometimes  break  when  the  depth  below  the  trough 
is  twice  the  height  of  the  wave.  Waves  may  thus  break  far  from 
the  shore  if  the  water  is  sufficiently  shallow.  "White-caps,"  seen 
on  the  crests  of  waves  in  deep  water  when  a  brisk  wind  is  blowing, 
are  simply  the  curling  over  and  lashing  into  foam  of  the  surface 
water  by  the  wind,  when  exposed  to  its  full  force  at  the  highest  part 
of  the  wave. 

The  Force  of  Waves. — The  force  with  which  waves 
break  against  the  shore  depends  upon  their  size,  which  in 
turn  depends  upon  the  force  and  direction  of  the  wind  and 
the  extent  of  its  contact  with  the  water  surface.  Thus, 
the  waves  are  stronger  when  the  wind  blows  toward  a 
coast  than  when  it  blows  seaward.  They  are  stronger 
upon  a  coast  facing  the  open  sea,  than  upon  one  protected 


WAVES  AND    TIDES.  1 25 

by  outlying  capes  or  islands.  As  the  prevailing  winds  in 
the  temperate  zones  are  toward  the  east,  it  is  generally 
true  that  in  those  zones  the  eastern  shores  of  the  oceans 
have  the  heavier  waves;  while  in  the  torrid  zone,  for  a 
similar  reason,  the  heavier  waves  are  on  the  western 
shores.  The  average  force  of  breakers  on  the  eastern 
shores  of  the  temperate  oceans  is  about  600  pounds  to 
the  square  foot  in  summer,  and  about  2,000  pounds  in 
winter,  when  the  winds  are  stronger. 

The  force  of  the  waves  at  the  eastern  end  of  Lake  Erie  has  been 
sufficient  to  tear  from  its  bed  in  the  masonry  at  Buffalo  harbor  a 
rock  weighing  half  a  ton,  and  after  moving  it  several  feet,  to  turn 
it  upside  down.  In  the  Shetland  Islands,  in  the  eastern  part  of  the 
Atlantic,  storm  waves  have  torn  blocks  of  stone  weighing  from  5  to 
19  tons  from  their  natural  beds  70  feet  above  sea-level,  and  carried 
them  many  feet  inland ;  while  at  Wick,  on  the  north-east  coast  of 
Scotland,  a  mass  of  masonry  weighing  1,350  tons  was  removed  en- 
tire from  the  end  of  the  breakwater  by  repeated  blows  of  storm 
waves  in  December,  1872,  and  a  mass  weighing  2,600  tons  was  simi- 
larly removed  in  1877. 

Tides. — An  observer  on  the  shore  soon  recognizes  the 
existence  of  other  wave-like  movements  of  the  sea,  quite 
different  from  that  of  wind  waves.  These  movements  are 
called  the  tides.  Tidal  waves  differ  from  wind  waves  in 
being  more  regular,  and  in  being  much  longer  in  propor- 
tion to  their  height.  They  are  so  long  that,  although 
they  travel  much  faster  than  wind  waves,  it  takes  about 
twelve  hours  for  one  to  travel  its  length.  They  are  so 
flat  that,  upon  the  open  coast,  they  never  form  breakers 
as  the  wind  waves  do. 

The  approach  of  the  crest  of  a  tidal  wave  is  indicated  on  the 
shore  by  the  gradual  rise  of  the  sea  surface,  and  the  flooding  of 
great  areas  of  low  coast ;  hence,  the  front  slope  of  the  tidal  wave  is 
called  flood  tide.  When  the  crest  arrives  at  the  shore,  the  sea  sur- 
face, having  nearly  reached  the  top  of  wharves  and  piers,  stops  ris- 
ing. It  is  then  high  tide.  In  a  short  time  the  sea  surface  gradually 
sinks,  and  the  water  slowly  flows  off  or  ebbs  away  from  the  sub- 


126 


PHYSICAL    GEOGRAPHY. 


Fig.  48.— High  and  Low  Tides. 


merged  flats,  giving  rise  to  the  name  ebb  tide  for  the  front  slope  of 
the  trough  of  the  tidal  wave.  The  arrival  of  the  trough  marks  low 
tide,  for  the  sea  surface  stops  sinking  and  soon  begins  to  rise  on 
the  front  slope  of  the  following  crest,  and  the  phenomena  are  re- 
peated. 

Tidal  Currents. — Tidal  waves  differ,  also,  from  wind 
waves  in  the  movement  of  the  water.  Wind  waves  cause 
no  current  by  which  a  floating  cork  is  carried  any  great 
distance  during  the  passage  of  a  wave.  Tidal  waves,  on 
the  contrary,  are  created  by  strong  currents,  in  which 
water  is  carried  forward  long  distances  on  the  crest  of 
the  wave,  and   backward  long  distances  in  its  trough. 


WAVES  AND   TIDES. 

The  movement  of  a  floating  cork  during  the  passage  of 
a  tidal  wave  describes  a  greatly  elongated  oval ;  thus,  sup- 
pose A,  B,  C,  etc.,  (Fig.  49),  to  be  equally  distant  points  in 
a  tidal  wave  advancing  to  the  right.  Suppose  a  cork  to  be 
floating  at  A.  The  points  b,  c,  d,  etc.,  indicate  the  posi- 
tion of  the  cork  when  the  corresponding  points  of  the 
wave  B,  C,  D,  etc.,  respectively  pass  under  it.  It  will  be 
noticed:  (1)  That  when  above  half  tide  level  the  cork 
moves  forward  ;  when  below  that  level,  it  moves  backward. 
(2)  That  at  half  tide  level  {A  and  /)  there  is  the  most 
rapid  rise  or  fall,  but  little  or  no  current ;  hence,  this  stage 
of  the  tide  is  called  slack  water.  (3)  That  near  high  tide 
(c,  d)  and  low  tide  [h,  i)  there  is  little  rise  or  fall,  but  the 
current  is  swiftest. 

Depths  of  Water  Disturbed  by  the  Tide, — 
While  wind  waves  disturb  only  a  thin  layer  of 
the  surface  water,  the  tidal  waves  are  caused  by 
movement  in  the  water  clear  to  the  bottom  of 
the  deepest  sea.  The  lesser  movement,  or  the 
rise  and  fall  of  the  tide,  decreases  from  the  sur- 
face to  the  bottom ;  but  the  greater  forward  and 
back  movements,  or  the  tidal  currents,  exist  at 
all  depths.  The  current  is  slower  near  the  bot- 
tom on  account  of  the  increased  friction. 

Length  and  Velocity  of  Tidal  Waves. — 
The  length  of  a  tidal  wave,  and  the  speed  at 
which  it  travels,  are  greater  in  deep  than  in  shal- 
low water.  Its  speed  is  such  that,  except  in  very 
shallow  water,  the  wave  travels  its  length  in  12 
hours  and  26  minutes,  or  about  half  a  day ;  hence, 
tides  are  semi-diurnal.  Really,  each  wave  re- 
quires 26  minutes  more  than  half  a  day  to  travel 
its  length ;  hence,  each  second  wave  arrives  at  the 
shore  about  52  minutes  later  than  the  wave  of 
the  day  before. 

In  water  three  miles  deep,  the  tidal  wave  is  nearly 
6^000  miles  long,  and  travels  nearly  500  miles  an  hour.  In 


128 


PHYSICAL   GEOGRAPHY. 


water  40  feet  deep  the  wave  is  but  300  miles  long,  and  travels  but 
25  «miles  an  hour.  At  high  and  low  tide  the  speed  of  the  tidal  cur- 
rents is  as  great  as  that  of  the  tidal  wave,  and  this  accounts  for  the 
energy  which  the  tidal  currents  display  in  piling  up  bars  or  scouring 
out  channels  about  the  mouth  of  harbors.  At  other  stages  of  the 
tide,  however,  the  tidal  currents  are  much  slower  than  the  tidal 
wave.  In  Fig.  49  it  may  be  noticed  that  the  cork  travels  only  from 
A  to/  while  the  wave  is  traveling  from  F  to/  But  in  the  diagram 
the  length  of  Af  is  greatly  exaggerated ;  in  the  deep  open  sea,  Af 
is  only  about  600  feet,  while  Ff  is  about  3,000  miles;  hence,  in 
such  a  locality  the  average  speed  of  the  tidal  current  is  but  100  feet 
an  hour,  though  the  wave  travels  500  miles  an  hour. 

Height  of  Tides. — In  the  deep  open  sea,  the  rise  and 
fall  of  the  tide  is  quite  insensible ;  it  is  probably  less  than 
two  feet.  When  the  tidal  wave  strikes  the  coast,  how- 
ever, its  slight  rise  and  fall 
becomes  perceptible  by  com- 
parison with  the  immovable 
land.  The  first  land  encount- 
ered by  the  tidal  wave  advancing 
from  seaward  are  the  ends  of  the 
capes  which  project  farthest  into  the 
sea.  At  these  points  the  rise  and  fall 
of  the  tide  is  generally  less  than  at  any 
other  point  of  the  coast,  for  as  the  cur- 
rents on  the  crest  of  the  tidal  wave  carry  the 
water  forward  into  the  bay  between  two  ad- 
jacent capes,  the  water  can  find  room  for  itself,  as  the  bay 
becomes  narrower  and  shallower,  only  by  rising  higher. 

Thus,  the  tidal  wave  of  the  Atlantic  (Fig.  50)  reaches  capes 
Florida  and  Hatteras,  and  Nantucket  Island  at  about  the  same  time. 
At  each  of  these  points  the  rise  and  fall  of  the  tide  is  between  one 
and  two  feet,  while  at  Savannah  and  at  Cape  May,  near  the  heads 
of  the  intervening  bays  or  bights,  the  height  of  the  tide  is  7  and  5 
feet  respectively.  This  effect  is  still  more  marked  in  the  Bay  of 
Fundy,  where,  entering  with  a  height  of  8  or  9  feet,  the  tidal  wave 
gradually  increases  to  a  height  of  over  40  feet  at  the  head  of  the  bay. 


Fig.  50. 


WAVES    AND    TIDES.  120, 

Duration  of  Flood  and  Ebb  Tides. — Upon  open 
coasts  the  front  and  rear  slopes  of  the  tidal  wave  are 
nearly  equally  steep,  and  the  trough  is  about  half-way 
between  the  two  crests ;  hence,  the  flood  and  ebb  tides 
are  of  equal  duration  —  each  about  six  hours.  In  com- 
paratively shallow  and  gradually  narrowing  bays,  the  slopes 
become  steeper,  for  the  wave  length  is  less  and  its  height 
greater;  and,  as  the  water  when  in  the  crest  of  the  wave, 
being  farther  from  the  bottom  and  less  retarded  by  fric- 
tion, moves  forward  faster  than  it  moves  backward  when  in 


the  trough,  the  wave  gradually  changes  its  shape  as  it  ad- 
vances, as  indicated  in  Fig.  51.  Therefore,  in  bays  and 
estuaries  the  flood  tide  is  generally  of  shorter  duration 
than  the  ebb  tide.  Thus,  at  the  mouth  of  Delaware  Bay 
the  flood  and  ebb  tide  each  continues  for  about  six  hours ; 
at  Newcastle,  Delaware,  the  tide  rises  for  5^  hours  and 
falls  for  about  7  hours,  while  at  Philadelphia  the  flood 
lasts  less  than  5  hours  and  the  ebb  more  than  7^. 

The  shape,  depth,  and  situation  of  some  estuaries  is  such  that  the 
front  slope  of  the  tidal  wave  becomes  so  steep  that  the  crest  falls 
forward  into  the  trough,  giving  the  flood  tide  the  form  of  a  breaker 
called  the  bore,  which  advances  up  the  estuary  at  the  great  speed  of 
the  wave.  In  this  case  the  flood  tide  is  but  momentary,  while  the 
ebb  lasts  about  twelve  hours.  In  the  deep  channel  of  the  estuary, 
the  tide  may  advance  as  a  steep  wave,  and  along  its  shallower  mar- 
gins as  a  breaker  or  bore.  The  bore  is  seen  at  the  head  of  the  Bay 
of  Fundy,  in  the  Hoogly  mouth  of  the  Ganges,  in  the  Dordogne  in 
France,  the  Severn  in  England,  the  Amazon  in  Brazil,  etc. 

Races. — Since  the  height  of  the  tidal  wave  depends  so 
largely  upon  the  shape  of  the  adjacent  shores  and  the 

P.  G.-S. 


I3O  PHYSICAL    GEOGRAPHY. 

depth  of  the  water,  the  same  wave  may  rise  to  very 
different  heights  in  neighboring  bays  on  the  same  coast. 
If  the  heads  of  these  bays  are  connected  by  a  narrow 
channel,  the  difference  in  water  level  in  the  two  bays  will 
give  rise  to  a  racey  or  strong  current,  in  the  channel,  flow- 
ing at  high  tide,  out  of  the  bay  in  which  the  tide  is  high- 
est. But  at  low  tide  the  water  is  lowest  in  this  bay; 
hence,  the  direction  of  the  race  is  reversed  with  each 
change  of  tide. 

Such  races  are  common  on  all  irregular  coasts,  especially  when 
fringed  with  islands.  Such  are  the  famous  "  Maelstrom "  among 
the  Lofoden  Islands,  the  currents  of  Pentland  Firth  north  of  Scot- 
land, and  those  of  Hellgate,  in  the  narrow  channel  between  Long 
Island  Sound  and  New  York  Bay.  If  the  waters  of  the  Sound 
could  be  separated  from  those  of  the  Bay  by  a  partition  at  this 
point,  the  water  at  high  tide  on  the  Sound  side  would  stand  5  feet 
higher,  and  at  low  tide  5  feet  lower  than  the  water  on  the  Bay  side. 

Cause  of  the  Tides. — The  great  regularity  in  the  re- 
currence of  the  tidal  wave  denotes  that  it  must  be  caused 
by  some  constant  and  regular  force;  and  the  fact  that  the 
water  to  its  greatest  depths  is  disturbed  by  it  indicates  that 
this  force  can  not  be  a  merely  superficial  one,  like  the 
wind.  A  force,  constant,  regular,  and  not  superficial  is 
found  in  the  mutual  attraction  of  gravitation  between  the 
earth,  the  moon,  and  the  sun,  and  it  is  the  peculiar  effect 
of  this  force  upon  the  liquid  sea  which  results  in  the  tidal 
wave.  As  the  effect  of  the  moon  is  usually  most  promi- 
nent, it  is  common  to  speak  of  the  tides  as  caused  solely 
by  the  moon.  They  are,  however,  always  modified  to  a 
greater  or  less  extent  by  the  sun  and  the  earth. 

Let  A  B  D  C  (Fig.  52)  represent  the  earth  and  let  M represent  the 
moon.  The  gravitation  of  the  moon  attracts  every  particle  of  the 
earth  with  a  force  which  varies  inversely  with  the  square  of  the  dis- 
tance between  the  particle  and  the  moon,  the  average  attraction  being 
that  exerted  on  the  particle  at  the  earth's  center,  E.  It  is  obvious 
that  the  moon's  attraction  on  the  side  of  the  earth  which  is  turned 


WAVES   AND   TIDES.  I3I 

towards  and  is  hence  nearer  to  the  moon  is  slightly  greater  than  the 
average,  while  the  attraction  on  the  more  remote  side  of  the  earth  is 
slightly  less  th  an  the  average.  There  is  consequently  a  tendency  for  the 
particles  on  the  side  of  the  earth  towards  the  moon  to  move  towards 
that  luminary,  and  for  the  particles  on  the  remote  side  of  the  earth  to 
move  away  from  the  moon .  The  solid  land  resists  this  tendency  to  move, 
but  the  liquid  sea  yields  to  it,  and  slow  movements  throughout  its  depth 
set  in  from  all  directions  toward  C  and  B,  by  which  the  sea  surface  is 
slightly  raised  to  H  and  F  at  points  on  opposite  sides  of  the  earth 
and  correspondingly  lowered  to  /  and  K  along  the  meridian  AED, 
half-way  between  //and  F. 

Owing  to  the  earth's  rota-  a  \ 

tion,  the  points  on  its  sur-  .f      '    .>*.  \ 

face,    towards    which    the      JsJL bV--f -fu\- 

tidal  currents  are  moving,  \  J/ 

are    constantly    changing,  ^^r^ 

and  after  about  six  hours,  A  lg"  52' 

and  D  occupy  the  positions  of  C  and  B,  I  and  K  being  then  ele- 
vated, and  H  and  F  depressed ;  that  is,  the  currents  in  the  sea 
which  before  moved  away  from  A  and  D  are  reversed,  and  now 
move  toward  these  points.  About  six  hours  later,  when  C  occupies 
the  position  of  B,  the  currents  are  again  reversed,  and  so  on.  This 
regular  reversal  of  the  slow  currents  of  the  sea,  which  for  about 
six  hours  advance  from  all  directions  toward  a  point,  and  then  for 
a  like  time  retreat  in  both  directions  away  from  this  point,  produces 
the  long,  low  tidal  wave,  whose  period  of  passage  is  always  about 
twelve  hours, — six  hours  for  the  crest  and  six  hours  for  the  trough. 
Tides  occur  later  each  day  because,  while  the  earth  is  making  a 
rotation,  the  moon  is  advancing  in  the  same  direction  in  her  orbit ; 
hence,  the  earth  has  to  make  a  little  more  than  a  complete  rota- 
tion to  present  the  same  point  of  its  surface  directly  to  the  moon. 
For  precisely  similar  reasons  the  sun  also  produces  a  tidal  wave, 
but  it  is  less  than  one  half  as  high  as  that  produced  by  the  moon ; 
for, although  the  sun's  attraction  is  much  greater  than  the  moon's, 
it  is  so  much  farther  away  that  the  diameter  of  the  earth  is  rela- 
tively insignificant,  and  his  attraction  on  opposite  sides  of  the  earth 
is  nearly  the  same.  Once  a  week,  however,  the  existence  of  the 
solar  tide  becomes  apparent.  The  moon  makes  a  complete  revolu- 
tion about  the  earth  in  four  weeks ;  twice  during  this  time — at  new 
and  full  moon — the  attraction  of  the  sun  and  of  the  moon  combine  to 


132 


PHYSICAL   GEOGRAPHY. 


/ 


FULL 

o- 

MOON 


;|R8T\_  ;-QUARTER 


N 


Neap)  Tide 


Tide     VI       i       V    Tide 


Neap  Tide 


\ 
\ 

NEW 

MOON 
/ 


THIRD/'     '\QUARTER 


Fig.  53- 


produce  an  unusually  high  tidal  wave  called  the  spring  tide.  At 
two  other  points  in  the  moon's  orbit — first  and  third  quarters — the 
crest  of  the  solar  occurs  in  the  trough  of  the  lunar  tidal  wave,  the 
combination  resulting  in  an  unusually  low  tidal  wave,  called  the 
neap  tide.     The  weekly  change  in  the  height  of  tidal  waves  is : 

Head  of  Bay  of  Fundy,     Spring  Tide,  50  feet.     Neap  Tide,  24  feet. 

Boston,  Mass.,    . 

New  York,  N  Y.,      . 

Cape  May,  N.  J., 

Cape  Hatteras,  N.  C, 

Savannah  Entrance,  Ga., 

Cape  Florida,  Fla.,    . 

San  Francisco,  Cal.,  . 

Astoria,  Oregon, 

Establishment  of  the  Port. — The  currents  moving 
under  the  attraction  of  the  moon  to  form  the  crest  of  the 
tidal  wave,  or  high  tide,  are  prevented  by  their  momen- 
tum from  stopping  immediately  when  under  the  attracting 
body.  Besides  this,  the  point  under  that  body  is  con- 
stantly advancing  westward  on  account  of  the  earth's  ro- 
tation ;  hence,  the  crest  of  the  wave  is  always  east  of  the 
meridian  under  the  moon.  The  momentum  depends  upon 
the  speed  of  the  water  and  its  amount,  i.  e. ,  upon  the  depth 
and  shape  of  the  bottom.  Hence,  though  the  interval  of 
time  is  always  the  same  between  the  passage  of  the  moon 


[1. 3" 

8.5  - 

5.4 " 

3.4  « 

6.0 " 

4-3  " 

2.2  "                      ' 

1.8  " 

8.0  " 

5.9  " 

1.8  " 

1.2  " 

4.3  " 

2.8  " 

7.4  " 

4.6  M 

WAVES   AND   TIDES.  1 33 

over  a  given  locality  and  the  arrival  of  high  tide,  this 
interval  varies  at  different  localities.  It  can  only  be  found 
by  observation,  and  when  found  is  called  the  establishment 
of  the  port. 

Thus,  the  establishment  of  the  port  of  Boston  is  u  hours,  27 
minutes;  of  New  York,  8  hours,  13  minutes;  of  Cape  May,  8  hours, 
33  minutes ;  of  Washington,  7  hours,  44  minutes  ;  of  Cape  Hatteras, 
7  hours,  4  minutes ;  of  Savannah  Entrance,  7  hours,  20  minutes. 
Each  indicates  the  time  intervening  between  the  moon's  transit  at 
that  point  and  the  arrival  of  the  succeeding  high  tide. 

Diurnal  Inequality  of  the  Tides. — The  inclination 
of  the  earth's  axis  to  the  orbit  of  the  moon  tends  to  pro- 
duce a  periodical  inequality  in  the  heights  of  the  two  daily 
tide  waves.  This  is  called  the  diurnal  ineqtiality.  Every 
two  weeks,  when  the  moon  is  over  the  equator,  both  waves 
are  of  equal  height,  but  in  the  intervening  time  one  wave 
tends  to  become  higher  than  the  other,  and  the  wave  that 
is  highest  during  the  fortnight  that  the  moon  is  north  of 
the  equator  is  lowest  during  the  following  fortnight  when 
the  moon  is  south  of  the  equator.  Owing  to  the  shape 
of  the  Atlantic,  the  effect  of  the  diurnal  inequality  is  not 
very  perceptible  in  that  ocean,  but  it  is  a  marked  feature 
on  the  Pacific  Coast,  and  its  effect  is  seen  in  the  Gulf  of 
Mexico,  where  it  causes  only  a  single  tide  a  day  to  be  per- 
ceptible, and  that  only  at  the  times  when  the  moon  is 
some  distance  north  or  south  of  the  equator. 

Fig.  54  (page  134)  shows  the  record,  during  a  fortnight,  of  tide 
guages  at  San  Francisco  and  the  mouth  of  the  Mississippi  River 
respectively.  Each  of  the  vertical  spaces  represents  one  day,  while 
the  rise  and  fall  of  the  tide  is  represented  by  the  curved  lines. 
It  is  seen  that  at  San  Francisco  there  are  two  tides  each  day,  but 
one  of  them  is  considerably  higher  than  the  other,  excepting  about 
the  time  when  the  moon  is  over  the  equator.  The  wave  (indi- 
cated by  dots  in  the  diagram)  which  is  highest  when  the  moon 
is  north  of  the  equator,  is  lowest  when  the  moon  is  south.  The 
semi-diurnal  tides   of   the  Atlantic  approach  the   Gulf  of  Mexico 


134 


PHYSICAL    GEOGRAPHY. 


by  the  two  channels  on  either  side  of  Cuba.  The  shape  and  depth 
of  these  channels  is  such  that  the  tide  wave  travels  through  them  at 
unequal  speed.  Thus,  the  crest  of  the  wave  from  one  channel,  and 
the  trough  of  the  wave  from  the  other,  enter  the  Gulf  simultane- 
ously. They  therefore  neutralize  each  other,  except  when  the  moon 
is  tar  from  the  equator,  and  the  diurnal  inequality  makes  the  waves 
of  unequal  heights.  At  such  times  this  difference  of  height  is  prop- 
agated through  the  Gulf  as  a  small  diurnal  wave,  while,  when  the 
moon  is  near  the  equator,  no  tides  are  perceptible. 


sf 


Ef 


3 


at 


d 


2 


^ 


^ 


^ 


(H 


b=i 


J — '/ 


SAN     FR 


ANC 


ISCQ 


Horizontal  lines  two  feet 


apart 


w-ey^RH — & 


m  fs-s-j  o  3 1  r  r  i — n  i  v  l  n 


Fig.  54- 


Tides  in  Lakes  and  Landlocked  Seas.— As  the 
force  which  produces  the  tides  is  universal,  it  affects  all 
water  surfaces  on  the  face  of  the  earth,  but  in  even  the 
largest  sheets  of  water  completely  cut  off  from  the  sea,  as 
the  Caspian  Sea  and  Lake  Superior,  the  length  of  the 
water  surface  is  so  insignificant  in  comparison  with  the 
length  of  the  tidal  wave  (half  the  circumference  of  the 
earth)  that  the  variation  of  level  in  the  small  part  of  the 
wave  formed  in  them  is  quite  imperceptible. 

Even  in  the  long  Mediterranean,  the  height  of  the  tidal  wave  on 
open  coasts  is  only  3  or  4  inches,  and  is  generally  obliterated  by  the 
wind.  The  converging  shores  render  the  tides  more  perceptible 
near  the  head  of  the  Adriatic  and  in  the  Straits  of  Messina,  where 
the  eddies  and  currents  "between  Scylla  and  Charybdis"  resemble 
those  of  Hellgate,  New  York. 


CHAPTER  X. 

CURRENTS    AND    DEPOSITS. 

They  thai  go  down  to  the  sea  in  ships,  that  do  business  in  great   waters ;    these 
see  the  works  of  the  Lord,  and  his  wonders  in  the  deep. — Psalm  cvii  :  23,  24. 

Currents. — In  addition  to  the  forward  and  back  move- 
ment of  the  water  in  wind  and  tidal  waves,  each  ocean  is 
traversed  by  systems  of  true  currents,  or  continuous  move- 
ments of  the  water  in  the  same  direction.  Several  causes 
combine  to  produce  these  continuous  currents;  the  princi- 
pal cause,  however,  is  the  inequality  in  the  density  of  the 
water  in  different  parts  of  the  sea,  arising  from  differences 
in  temperature  and  saltness. 

Effect    of   Temperature. — Since   water   expands  and 

becomes   less  dense  when  heated,  the  surface  of  the  sea 

stands  somewhat  higher  near  the  tropics  than  in  the  frigid 

zones,    where   the   water   is   400  or   500  colder.     Gravity 

gives  the  surface  water  a  tendency  to  flow  down  the  gentle 

slope  thus  formed,    from  the  tropics  toward  the  nearest 

pole,  while  the  increased  pressure  upon  the  deeper  water 

caused  by  the  arrival  in  higher  latitudes   of  the  surface 

water  from  the  tropics,  gives  the  deeper  water  a  tendency 

to  flow  back  toward  the  tropics.     These  movements   are 

facilitated  by  the  fact  that  the  deep  polar  water  is  more 

salty,   colder,  and  hence  heavier  than  the  deep  water  of 

lower   latitudes,    while    the    polar    surface   water,    though 

colder  than  the  surface  water  at   the   tropics,    is  not  so 

dense   or   heavy,  because  the  melting  ice  renders  it  less 

salty. 

das) 


I36  PHYSICAL  GEOGRAPHY. 

Effect  of  Saltness. — The  constant  trade  winds  start 
from  the  tropics  very  dry,  but  arrive  at  the  equatorial 
calms  saturated  with  vapor.  Rising  over  these  calms,  the 
vapor  condenses  and  causes  almost  constant  rains.  Since 
vapor  is  perfectly  fresh,  the  region  of  the  sea  from  which 
it  is  taken  is  left  more  salty,  and  the  region  on  which  it  is 
precipitated  as  rain  is  made  less  so ;  hence,  the  surface 
water  near  the  tropics  is  more  salty  and  heavier  than  that 
in  the  equatorial  calms.  The  tropical  surface  water,  heavy 
through  its  excess  of  salt,  can  not  sink  far  because  the 
deeper  water  is  equally  heavy  from  its  lower  temperature ; 
it  consequently  moves  toward  the  equatorial  calms  to  dis- 
place the  lighter  water  there. 

Thus,  the  varying  temperature  and  saltness  of  the 
water  in  different  parts  of  the  sea  give  the  surface  water  a 
general  tendency  to  move  toward  the  equator  in  the  torrid 
zone,  and  toward  the  poles  in  the  temperate  and  frigid 
zones ;  and  to  the  deeper  water,  in  all  zones,  a  general  ten- 
dency to  move  toward  the  equator. 

The  direction  in  which  ocean  currents  flow  is  greatly 
modified,  however,  by  (1)  the  rotation  of  the  earth ;  (2) 
the  configuration  of  the  coast  and  sea  bottom;  (3)  by 
other  currents;  and  (4)  by  the  winds. 

The  rotation  of  the  earth  gives  to  moving  water,  as  it  does  to  mov- 
ing air  (page  79),  a  tendency  to  turn  out  of  a  straight  course.  In  the 
northern  hemisphere  it  tends  to  turn  to  the  right,  and  in  the  southern 
hemisphere  to  the  left.  This  tendency  affects  moving  water  at  all 
depths,  and  increases  from  the  equator  to  the  poles. 

The  coast  of  an  ocean  deflects  currents  at  all  depths  which  flow 
against  it.  If  the  current  strikes  the  shore  almost  at  right  angles, 
part  of  it  is  deflected  to  the  right  and  part  to  the  left.  The  config- 
uration of  the  sea  bottom  influences  the  direction  of  deep  water 
currents  in  the  same  way,  for  as  the  heaviest  water  sinks  to  the  bot- 
tom, this  water,  when  moving  as  a  current,  can  not  rise  through  the 
lighter  water  above  to  pass  over  submarine  banks  or  ridges,  which 
therefore  deflect  currents  in  the  deeper  water. 


CURRENTS    AND    DEPOSITS.  1 37 

A  current  meeting  another  at  any  angle  deflects  it,  and  is  itself 
deflected  to  the  right  or  left,  or  in  both  directions,  according  to  the 
angle  of  meeting  and  the  respective  strength  of  the  currents. 

The  friction  of  the  wind  on  the  sea  surface  tends  to  move  the 
wate;  in  the  direction  of  the  wind.  If  the  wind  moves  in  the  same 
direction  as  the  current,  it  tends  to  make  the  current  move  faster ;  if 
it  blows  obliquely  across  the  current,  it  tends  to  deflect  the  current ; 
if  it  blows  against  the  current,  it  tends  to  check  and  may  even  re- 
verse for  the  time  being  a  gentle  flow.  The  effect  of  the  friction  of 
the  wind  is  always  superficial,  however ;  Professor  Ferrel  estimates 
that  its  influence  immediately  beneath  the  surface  is  less  than  y^th 
of  that  arising  from  unequal  density  of  the  water. 

Surface  Currents. — In  consequence  of  these  modify- 
ing influences,  the  general  movement  of  the  surface  water 
in  each  of  the  oceans  is  outward  and  around  the  tropical 
regions  where  the  surface  water  is  densest  and  heaviest, 
the  currents  thus  forming  great  outward  moving  whirls 
similar  to  anticyclones  in  the  atmosphere.  The  general 
movement  of  these  whirls  is  westward  on  either  side  of 
the  equatorial  calms,  away  from  the  equator  in  the  western 
part  of  the  oceans,  eastward  between  latitudes  400  and 
6o°,  and  toward  the  equator  in  the  eastern  part  of  the 
oceans.  In  the  narrow  region  of  equatorial  calms  a 
"counter-current"  moves  eastward  in  all  the  oceans.  Be- 
yond latitude  500,  in  the  southern  hemisphere,  there  is 
but  little  land  to  deflect  the  movement  of  the  surface  water 
from  its  general  easterly  course  around  the  globe.  In  the 
higher  latitudes  of  the  northern  oceans,  however,  and  es- 
pecially in  the  Atlantic,  with  its  Arctic  extension,  the 
easterly  moving  water  between  500  and  6o°  latitude  en- 
counters the  west  coasts  of  the  continents.  Part  of  it  is 
deflected  northwardly  along  these  coasts,  thus  causing  a 
southward  return  current  along  the  east  coasts  of  the  con- 
tinents in  these  higher  latitudes. 

While  the  movement  of  surface  currents  is  generally  similar  in 
all  the  oceans,  differences  in  the  shape  and  extent  of  the  coast-line, 


RELATIVE   DENSITY 

of  the  Surface  Water, 

THE   SURFACE    CURRENTS 

and  Equatorial  limits  of 

*  FLOATING    ICE 

in  the  various  parts  of  the  Oceans 

M 


ific  Gravity  le  n  than  1.025 

"     1.025    to      1.026 

"     1.026   to      1.027 

"     1. 027    to       1.02 

"      more  than     1.02 


with  a  Sp.  Gr.  of  1.02 
tons   hpavier  than   an 
with  a 


savier  man   an  < 
Sp.  Gr.  of  1.02,1 


£. 


(138) 


1  *{? 


^x 


U39) 


I4O  PHYSICAL    GEOGRAPHY. 

and  in  other  modifying  influences,  cause  local  peculiarities  in  the 
speed,  temperature,  and  constancy  of  the  currents  in  each  ocean,  and 
these  vary  at  different  seasons  of  the  year. 

The  Gulf  Stream. — The  western  part  of  the  tropical  whirl  in  the 
Atlantic  is  called  the  Gulf  Stream,  and  off  the  southern  coast  of 
Florida  it  is  made  one  of  the  most  rapid  of  ocean  currents  by  the 
peculiar  configuration  of  the  coast  in  that  vicinity.  The  equatorial 
current  enters  the  Gulf  of  Mexico  through  the  broad,  deep 
Yucatan  Channel,  and  forces  an  equal  amount  of  water  to  flow 
out  through  the  Strait  of  Florida.  This  strait  being  relatively 
shallow  and  narrow,  the  outflow  is  more  rapid  than  the  inflow. 

The  Kuro  Siwo,  or  Black  Stream,  as  the  corresponding  current 
in  the  western  part  of  the  Pacific  is  called,  though  a  well  marked 
current,  is  not  so  strong  or  well  marked  as  the  Gulf  Stream  because 
of  the  chain  of  islands  which  border  the  east  Asiatic  coast — For- 
mosa, Japan,  etc., —  among  which  part  of  the  current  is  deflected 
from  its  regular  north-easterly  course,  and  also  because  of  the 
strong  winter  monsoon  of  that  region,  which  at  that  season  diverts 
part  of  this  current  to  the  south-west  through  the  Malay  Archipelago 
into  the  Indian  Ocean. 

In  the  North  Indian  Ocean  the  effect  of  the  monsoons  upon  the 
surface  currents  is  very  marked.  In  January  the  north-east  mon- 
soon strikes  the  northern  part  pf  this  ocean  as  a  dry  land  wind,  and 
evaporating  water  rapidly,  renders  the  surface  water  salty  and 
heavy.  Aided  by  the  friction  of  the  wind,  the  water  flows  south- 
westward  toward  the  lighter  and  fresher  water  in  the  region  of 
equatorial  calms  and  rains.  In  July,  however,  the  south-west  mon- 
soon, rendered  damp  near  the  equator,  pours  down  fresh  water  on 
the  northern  part  of  the  ocean,  and  the  Salter  and  heavier  equatorial 
water,  aided  by  the  friction  of  the  winds,  flows  north-eastward  and 
along  the  coast  into  the  China  Sea  to  augment  the  strength  of  the 
Kuro  Siwo. 

Effect  of   Surface  Currents  upon  Temperature. — 

The  water  composing  the  surface  currents  is  warmed  in 
equatorial  regions,  and  arrives  in  higher  latitudes  with  a 
higher  temperature  than  the  sun  is  able  to  maintain  at  that 
latitude.  It  therefore  cools  by  imparting  its  excess  of  heat 
to  the  water  below  and  the  air  above,  and  arrives  at  the 
equator  again,  in  the  eastern  part  of  the  oceans,  cooler 


CURRENTS    AND    DEPOSITS. 


I4I 


than  the  equatorial  air  and  water.  These  are  slightly 
cooled  by  imparting  heat  to  the  cooler  current,  and  aiding 
the  sun  to  warm  it  again  during  its  westward  flow  across 
the  ocean.  Thus,  all  currents  tend  to  moderate  the  tem- 
perature of  the  region  they  traverse ;  if  they  come  from  a 
warmer  region,  they  tend  to  raise  the  temperature ;  if 
from  a  colder  region,  to  lower  it.  It  has  already  been 
stated  (page  61)  that  about  one  half  of  the  heat  received 
by  the  whole  torrid  zone  is  carried  by  ocean  currents  into 
colder  latitudes. 


Fig.  55. 


Since  the  currents  between  the  equator  and  about  450  latitude 
move  from  the  equator  in  the  western  part  of  the  oceans  and 
toward  the  equator  in  the  eastern  part,  it  follows  that  the  western 
part  of  the  oceans  in  these  latitudes  contains  a  greater  amount  of 
warm  water  than  the  eastern  part.  This  is  well  shown  in  the  tem- 
perature sections  across  the  Atlantic  and  Pacific  oceans  along  the 
parallel  of  350  north  latitude  (Fig.  55).  A  temperature  higher  than  590 
extends  to  a  depth  of  over  300  fathoms  in  the  western  part  of  the 
Atlantic,  but  to  scarcely  100  fathoms  in  the  eastern  part.  Owing  to 
the  less  relative  strength  of  the  warm  current  in  the  Pacific  (the 
Kuro  Siwo),  and  the  greater  size  of  that  ocean,  the  isotherm  of  590 
lies  at  a  less  depth  in  the  Pacific  than  in  the  Atlantic ;  but  while  it 
reaches  a  depth  of  nearly  150  fathoms  in  the  western  part  of  the 
ocean,  it  rises  to  within  less  than  50  fathoms  in  the  eastern  part.  In 
higher  latitudes,  on  account  of  the  reversed  directions  of  the  cur- 


142  PHYSICAL    GEOGRAPHY. 

rents,  the  waters  of  the  north  Atlantic  and  Pacific  are  warmer  in  the 
eastern  than  in  the  western  parts  of  these  oceans.  In  the  Atlantic  a 
narrow  branch  of  the  cold  polar  current  follows  the  American  coast 
southward  to  the  latitude  of  the  Carolinas,  forming  a  "cold  wall" 
of  water  between  the  warm  Gulf  Stream  and  the  shore.  The  cold 
southwardly  flowing  current  in  the  high  latitudes  of  the  western 
Atlantic  brings  vast  numbers  of  icebergs  down  to  the  neighborhood 
of  Newfoundland,  where,  meeting  the  warmer  waters  of  the  Gulf 
Stream,  the  bergs  melt  rapidly  and  deposit  their  load  of  rocky 
material.  This  deposit,  gathering  through  untold  ages,  has  pro- 
duced the  shoals  of  that  vicinity  called  the  Newfoundland  Banks. 

Deep  Sea  Currents. — The  systems  of  surface  currents 
affect  but  a  comparatively  thin  layer  of  water.  They 
seldom  extend  to  a  greater  depth  than  a  few  hundred 
fathoms.  The  movements  of  the  great  mass  of  sea-water 
below  this  depth  can  not  be  directly  observed,  but  pro- 
gressive movements  or  currents  in  it  are  known  to  exist 
because  of  its  temperature. 

Only  a  comparatively  thin  layer  of  surface  water  is  directly 
affected  by  the  heat  of  the  sun ;  this  water,  however,  affects  the 
temperature  of  the  layer  beneath,  and  this  of  the  still  deeper  water ; 
hence,  if  there  were  no  currents  the  temperature  of  the  sea  in  any 
latitude  would  eventuallyjpecome  uniform  at  all  depths ;  that  is,  it 
would  be  about  8o°  from  surface  to  bottom  at  the  equator,  and  about 
290  in  polar  regions.  The  fact  that  at  greater  depths  than  800 
fathoms  the  water,  even  under  the  equator,  never  has  a  temper- 
ature higher  than  400,  proves  that  the  deeper  water  is  constantly 
being  replaced  by  cold  water  before  it  can  be  warmed  by  the  warmer 
water  above.  Now,  from  the  equator  nearly  to  the  polar  circles  the 
temperature  of  the  surface  water  is  higher  than  400  (see  Fig.  46), 
and  tends  to  raise  the  temperature  of  the  deeper  water  above  that 
point ;  hence,  the  deep  water  that  arrives  in  equatorial  latitudes  with 
a  temperature  less  than  400  must  come  from  the  frigid  zones. 

Direction  and  Velocity  of  Deep  Sea  Currents. — 
The  configuration  of  the  coasts  and  bottom  of  the  oceans 
indicates  that  by  far  the  larger  part  of  the  cold  deep  sea 
currents,  even  in  the  northern  hemisphere,  comes  from  the 
Antarctic  Ocean ;  but  they  must  move  with  extreme  slow- 


CURRENTS   AND    DEPOSITS. 


H3 


ness,  otherwise  the  finely  powdered  material  which  com- 
poses the  bottom  of  the  ocean  would  be  swept  away  by 
them. 

The  only  places  where  the  Arctic  water  could  flow  south  are  be- 
tween Europe  and  America  into  the  Atlantic,  and  through  Behring 
Strait  into  the  Pacific.  But  a  submarine  ridge,  on  which  Iceland 
stands,  extends  entirely  from  Europe  to  America.  It  rises  every- 
where to  within  500  fathoms  of  the  surface,  and  therefore  prevents 
the  deeper  water  of  the  Arctic  from  entering  the  Atlantic.  East  of 
Iceland  all  the  water  above  the  top  of  the  ridge  is  warmer  than  400, 
and  moves  northwardly ;  hence,  it  is  only  in  the  comparatively 
narrow  channels  between  Iceland  and  Labrador,  and  in  the  entirely 
insignificant  Bering  Strait,  that  any  of  the  cold  surface  water  of  the 
Arctic  escapes  southwardly. 

Theory  Confirmed. — The  theory  that  the  low  temper- 
ature of  the  deeper  water  in  the  sea  is  produced  by  cold 
under-currents  from  the  polar  regions,  is  confirmed  by  the 
comparatively  high  temperature  of  the  deeper  water  in 
regions  which  such  under-currents  can  not  enter  on  account 
of  an  intervening  submarine  ridge. 
The  Mediterranean  and  Caribbean 
seas  are  such  regions,  as  well  as" 
several  of  the  east  Asiatic  seas,  and 
perhaps  the  whole  north  Pacific 
Ocean. 

As  indicated  in  Fig.  56,  the  tempera- 
ture of  the  Atlantic  opposite  the  Strait  of 
Gibraltar  falls  continuously  from  over 
700  at  the  surface  to  about  370  at  the 
bottom  in  2,200  fathoms.  The  Strait  of 
Gibraltar,  with  a  depth  of  less  than  200  lg- 

fathoms,  admits  to  the  Mediterranean  no  water  colder  than  550. 
The  deep  basin  of  the  Mediterranean  is  thus  filled  with  water  no 
colder  than  that  which  enters  in  the  lower  part  of  the  inlet  current 
from  the  Atlantic  with  a  temperature  of  550.  Hence,  the  tempera- 
ture of  the  Mediterranean  falls  continuously  only  from  the  surface 
to  the  depth  of  the  bottom  of  the  inlet  current  (about  125  fathoms), 


144  PHYSICAL    GEOGRAPHY. 

beyond  which,  clear  to  the  bottom  (over  2,000  fathoms  in  some 
places),  the  temperature  of  the  water  is  uniform  at  550.  A  uniform 
temperature  of  39^°  was  long  since  observed  in  the  Caribbean  Sea 
and  the  Gulf  of  Mexico  at  all  depths  greater  than  about  1,000 
fathoms.  The  existence  of  a  channel  to  the  Atlantic,  with  a  depth 
of  about  1,000  fathoms,  was  therefore  inferred,  although  all  known 
channels  were  much  shallower.  In  1884,  however,  a  channel  be- 
tween Puerto  Rico  and  Santa  Cruz  was  discovered  having  a  depth 
of  926  fathoms,  and  a  bottom  temperature  of  39^°.  The  deep  seas 
of  the  Malay  Archipelago  have  uniform  temperatures  below  depths 
varying  from  400  to  900  fathoms,  from  which  it  is  inferred  that  each 
of  these  seas  is  inclosed  by  a  submarine  ridge,  whose  lowest  point 
corresponds  to  the  depth  at  which  the  uniform  temperature  begins. 
All  temperature  observations  in  the  Pacific  north  of  a  line  from 
northern  Chile  to  China,  indicate  a  uniform  temperature  below  a 
depth  of  1,500  fathoms,  while  to  the  south  of  this  line  the  tempera- 
ture decreases  constantly  to  the  bottom.  Hence  the  inference  that 
a  submarine  ridge,  rising  to  within  at  least  1,500  fathoms  of  the  sur- 
face, unites  South  America  with  Asia,  and  prevents  the  cold  bottom 
water  of  the  Antarctic  from  entering  the  North  Pacific. 

Currents  between  the  open  ocean,  and  nearly  in- 
closed arms  of  the  sea,  depend  to  a  great  extent,  like  the 
surface-currents  in  the  open  ocean,  upon  evaporation  and 
precipitation.  If  more  water  falls  in  the  basin  of  the  par- 
tially inclosed  sea  than  is  evaporated  from  its  surface — as 
in  New  York  Bay  and  the  Baltic  and  Black  seas — its  sur- 
face tends  to  rise  higher  than  that  of  the  ocean,  and  a 
current  from  the  bay  or  sea  into  the  ocean  is  the  result. 
If,  however,  less  water  is  precipitated  in  the  basin  of  the 
sea  than  is  evaporated  from  its  surface,  as  the  Mediter- 
ranean and  Red  seas,  the  surface  tends  to  fall  below  the 
ocean  level,  and  a  current  from  the  ocean  into  the  sea  is 
the  result. 

In  the  latter  case,  since  only  fresh  water  is  removed  by  evapora- 
tion, and  since  the  level  of  the  inclosed  sea  is  maintained  by  a  flow 
of  salt  water  from  the  ocean,  the  tendency  is  for  the  sea  to  become 
constantly  more  salty.  Both  the  Mediterranean  and  Red  seas  are 
more  salty  than  the  ocean,  but  their  water  does  not  appear  to  in- 


CURRENTS    AND    DEPOSITS. 


145 


crease  in  saltness.  The  tendency  to  increase  in  saltness  must  there- 
fore be  counteracted  by  a  current  which  carries  just  enough  of  the 
excessively  salty  water  of  the  inclosed  sea  into  the  ocean  to  prevent 
a  constant  increase  of  saltness.  As  the  inclosed  sea  water  is  heavier 
than  the  fresher  ocean  water,  the  outflowing  current  occupies  the 
bottom,  and  the  inflowing  current  the  top  of  the  channel  by  which 
the  sea  communicates  with  the  ocean.  Therefore,  the  uniform  tem- 
perature in  the  Mediterranean  begins,  hot  at  the  depth  of  the  Strait 
of  Gibraltar,  but  at  the  depth  of  the  bottom  of  the  inflowing  current ; 
for  the  outflowing  under-current  is  as  effectual  a  barrier  to  the  en- 
trance of  colder  ocean  water  as  the  submarine  ridge  itself. 

Deposits  of  the  Sea. — In  addition  to  the  solid  matter 
dissolved  in  sea-water,  which  gives  it  the  salty  and  bitter 
taste,  there  is  always  a  quantity  of  solid  matter,  in  coarser 
or  finer  grains,  which  is  gradually  sinking  through  sea-water, 
and  forming  a  deposit  on  the  bottom.  This  sediment  is 
derived  chiefly  from  three  sources:  (1)  the  continents,  (2) 
the  animals  and  plants  which  inhabit  the  sea,  and  (3)  the 
material  ejected  from  volcanoes.  According  as  the  bot- 
tom in  any  region  of  the  sea  is  composed  principally  of 
matter  derived  from  one  or  other  of  these  sources,  the  de- 
posit is  called  continental,  organic,  or  red  clay. 

Continental  Deposits  cover  about  two  fifths  of  the 
sea  bottom.  They  form  the  sea  bottom  for  a  distance  of 
300  or  400  miles  from  the  shores  of  all 
the  continents  and  continental  islands, 
and  extend  completely  across  the  polar 
oceans  and  all  partially  inclosed  seas. 
They  consist  of  variously  colored  muds, 
composed  principally  of  very  minute 
rounded  fragments  of  the  rocks  which 
constitute  the  land.  These  muds  also 
contain  organic  remains  and  volcanic  minerals. 

Fragments  from  continental  deposit,  magnified  ten  times,  are 
shown  in  Fig.  57.  Pieces  of  the  rocky  coast  are  being  constantly 
broken  off  and  ground  to  powder  by  the  force  of  the  waves,  while 


ssa 


*'ig-  57- 


146 


PHYSICAL    GEOGRAPHY. 


Fig.  58.— Globigerina  Ooze. 
{Magnified  13  times.) 


Fig.  59.— Pteropod  Ooze. 
{Magnified  3  times.) 


the  water  of  every  stream  is  more  or  less  muddy,  according  as  its 
current  is  carrying  or  rolling  along  a  greater  or  less  amount  of 
rocky,  earthy,  or  other  continental  material.  The  larger  fragments 
broken  off  by  the  waves  or  brought  down  by  the  rivers,  sink  to  the 
bottom  near  the  shore  of  the  ocean,  to  be  rolled  about  and  ground 
finer  by  the  waves;  but  the  finer  pieces  sink  more  slowly,  and  are 
carried  farther  away  by  the  ocean  currents. 
It  is  only  in  exceptional  cases,  however,  such 
as  floating  ice,  etc.,  that  even  the  most 
minute  fragments  are  carried  more  than 
300  or  400  miles  before  they  settle  to  the 
bottom. 


Fig.  60. 
Radiolaria  Ooze. 

{Magnified  50  times.) 


Organic  Deposits  differ  from  con- 
tinental deposits  in  containing  no  re- 
mains of  continental  rocks.  An  organic 
deposit  constitutes  the  bottom  in  such 
portions  of  the  sea  as  lie  beyond  the 
limits  of  the  continental  deposits,  and 
have  a  depth  less  than  2,900  fathoms. 
It  is  a  soft,  fine  mud,  or  ooze,  composed 
principally  of  the  shells  or  stony  frame- 
work of  minute  organisms  which  live 
near  the  surface  of  torrid  and  temper- 
ate seas.  It  is  called globigerina,  pteropod, 
or  radiolaria  ooze,  if  the  shells  of  these 
animals  respectively  are  most  numerous,  or  diatom  ooze  if 
the  stony  frustules  of  this  plant  are  in  excess. 


Fig.  61.— Diatom  Ooze. 

{Magnified  ioo  times.) 


CURRENTS    AND    DEPOSITS. 


147 


Fig.  62. 


The  shells  or  stony  frame-work  of  sea  organisms  are  largely  com- 
posed of  carbonate  of  lime,  extracted  from  sea-water  during  the  life 
of  the  organism ;  at  death  these  stony  structures  begin  to  sink,  and 
are  slowly  dissolved  again  by  the  sea-water.  If  the  sea  is  deeper 
than  about  2,900  fathoms,  the  calcareous  or  limy  portions  are  entirely 
dissolved  before  they  reach  the  bottom ;  but  if 
of  less  depth,  fragments  of  them  may  reach 
the  bottom  and  be  covered  up  and  protected 
by  following  pieces.  Hence,  organic  deposits 
are  always  calcareous,  sometimes  being  nearly 
pure  carbonate  of  lime. 

Red  Clay  Deposits  differ  from  con- 
tinental and  organic  deposits  in  the 
general  absence  of  continental  debris 
and  calcareous  organic  remains.  The  red  clay  covers  the 
sea  bottom  beyond  the  limit  of  the  continental  deposits,  in 
depths  greater  than  2,900  fathoms.  It  is  a  stiff  clay, 
greasy  to  the  touch,  plastic  when  wet,  but  very  hard  when 
dry.  It  is  composed  almost  exclusively  of 
the  minerals  which  are  found  in  volcanic 
rocks.  Most  of  the  minute  mineral  frag- 
ments found  in  it  are  sharp  and  angular,  (Fig. 
62,  magnified  ioo  times,)  in  marked  contrast 
Fig.  63.  tQ  tjie  rouncje(]  fragments  of  the  continental 

deposits.  The  surface  of  the  deposit  is  strewn  with  pieces 
of  pumice  stone,  minute  particles  of  magnetic  iron  of 
meteoric  origin,  and  with  great  numbers  of  the  hardest 
bones  of  sea  animals,  as  the  ear-bones  of  whales  and  the 
teeth  of  sharks,  some  of  which  belong 
to  species  once  plentiful  but  long  since 
extinct.  The  older  bones  are  covered 
with  a  thick  coating  of  oxide  of  manga- 
nese, while  the  bones  of  more  recent 
species  are  quite  clean.  Figure  63 
shows  the  incrusted  tooth  of  a  shark,  Fie-  ^ 

and  Figure  64  the  incrusted  ear-bone  of  a  whale.. 


I48  PHYSICAL    GEOGRAPHY. 

The  volcanic  materials  which  compose  the  red  clay  are  derived 
from  pumice  stone,  which  is  so  light  that  it  floats  for  great  distances 
before  sinking ;  from  volcanic  ashes,  which  are  carried  to  great  dis- 
tances by  the  winds ;  and  from  volcanic  lavas  and  tufas  laid  down 
directly  on  the  sea  bottom.  All  these  volcanic  products  are  rich  in 
the  minerals  of  which  clay  is  composed,  and  these  minerals,  being 
liberated  by  the  chemical  action  of  the  sea-water,  reunite  in  the  pro- 
portions to  form  the  red  clay. 

What  the  Deposits  Teach. — The  character  of  the 
various  deposits  goes  far  toward  confirming  the  belief  that 
the  present  ocean  basins  have  been  depressed  regions,  and 
that  the  present  continents  have  been  elevated  regions  con- 
tinuously, from  a  very  early  period  of  the  earth's  history ; 
but  while  the  present  regions  of  organic  and  red  clay 
deposits  have  always  been  covered  by  water,  the  marginal 
region  of  continental  deposit,  as  well  as  the  present  land, 
have  been  subjected  to  many  upward  and  downward  move- 
ments, by  which  large  areas  of  each  have  been  alternately 
raised  above  and  lowered  beneath  the  surface  of  the  sea. 
Thus,  the  continents  and  the  oceans,  though  constantly 
varying  somewhat  in  shape  and  size,  have  always  main- 
tained their  present  general  arrangement. 

The  great  antiquity  of  the  red  clay  deposit,  and  the  extreme  slow- 
ness with  which  it  collects,  are  indicated  by  the  abundance  of  me- 
teoric fragments,  and  whales'  and  sharks'  bones,  many  of  them  of 
,  extinct  species  and  deeply  incrusted,  which  are  found  on  the  surface 
of  this  deposit.  Great  numbers  of  fragments  and  bones  probably 
settle  upon  the  other  deposits  also,  but  are  covered  up  and  buried  in 
the  more  rapidly  accumulating  continental  and  organic  debris.  Most 
of  the  rocks  of  the  continents  bear  evidence  of  being  a  hardened  sea 
deposit  very  similar  to  the  continental  deposits  now  forming,  but  no 
rocks  have  been  found  similar  to  organic  and  red  clay  deposits  of 
the  deep  open  ocean.  From  this  it  is  inferred  that  most  of  the  conti- 
nental rocks  were  formed  as  a  continental  deposit  beneath  the  sur- 
face of  the  sea  like  the  present  continental  deposits,  at  no  great 
distance  from  the  land,  and  afterward  elevated  above  sea-level. 
Such  gradual  deration  or  subsidence  of  coast  regions  is  now  in 
actual  progress  in  many  parts  of  the  earth. 


PART   IV.— THE  LAND. 


CHAPTER  XL 

DIVISIONS    OF   THE    LAND. 

And  God  said.  Let  the  waters  under  the  heaven  be  gathered  together  unto  one 
place,  and  let  the  dry  land  appear:  and  it  was  so. — Genesis  i:  9. 

Comparative  Smoothness  of  the  Earth's  Surface. — 
In  speaking  of  the  earth  as  a  whole,  its  solid  surface  was 
considered  as  being  perfectly  smooth,  and  in  comparison 
with  the  vast  dimensions  of  the  planet,  the  irregularities 
of  its  surface  are  insignificant.  These  irregularities  are  of 
vast  importance,  however,  since  they  cause  the  division  of 
the  surface  of  the  earth  into  areas  of  sea  and  land. 

The  relative  insignificance  of  the  surface  irregularities  can  be 
appreciated  from  the  diagram  on  the  next  page  (Fig.  65),  in  which 
the  heights  and  depths  have  been  exaggerated  ten  times. 

The  Land. — The  tops  of  the  highest  irregularities  on 
the  earth's  surface  protrude  above  the  surface  of  the  sea 
and  form  land.  The  total  land  area  of  the  world  is  about 
52,500,000  square  miles,  and  constitutes  but  little  more 
than  one  fourth  (26^%)  of  the  surface  of  the  planet. 

The  Level  of  the  Sea. — The  sea  has  a  smoother  sur- 
face than  the  solid  globe.  Though  always  slightly  rough- 
ened by  waves,  it  never  varies  more  than  a  few  feet  from 
perfect  smoothness.  Its  mean  height  when  half-way  be- 
tween low  and  high  tide  is  usually  adopted  as  the  base, 

P.G.-9.  (149) 


I50  PHYSICAL  GEOGRAPHY. 


Fig.  65. — The  Proportional  Roughness  of  the  Earth's  Surface  Exaggerated 
Ten  Times. 

called  sea-level,  from  which  all  differences  of  elevation  in 
the  earth's  solid  surface  are  measured. 

Regions  of  Elevation  and  Depression. — The  mean 
height  of  the  land  above  sea-level  is  a  little  less  than  one 
half  a  mile.  As  the  mean  depth  of  the  sea  is  2*4  miles, 
the  total  mean  height  of  the  land  above  the  sea  floor  is 
about  3  miles.  An  elevation  half  as  great  (that  is,  \]/2 
miles  above  the  sea  floor),  may  therefore  be  taken  to 
divide  the  regions  of  elevation  in  the  earth's  crust  from 
the  regions  of  depression.  In  other  words,  not  only  the 
land,  but  all  parts  of  the  sea  bottom  on  which  the  water 
is  less  than  1  mile  deep,  are  to  be  considered  as  regions 
of  elevation,  while  only  the  sea  bottom  at  greater  depths 
is  to  be  considered  the  region  of  depression.  This 
region  of  depression  is  shown  in  solid  black  in  the  map 
bn  pages  152  and  153;  the  regions  of  elevation  are 
shaded  or  are  left  white. 

Region  of  Elevation. — The  map  shows  that  there  is 
but  one  great  region  of  elevation.  It  extends  entirely 
across  the  northern  hemisphere,  and  at  three  places  pene- 


DIVISIONS    OF    THE    LAND. 

urates  the  southern  hemisphere  to  about  400  south  latf5"^ 
tude.  The  height  of  this  continuous  region  of-  elevation 
is  not  uniform ;  at  certain  localities  it  does  not  reach  quite 
to  the  level  of  the  sea,  but  enough  of  it  protrudes  above 
the  sea  to  constitute  almost  all  (T9o9o3otns)  °f  tne  lanc*  on  the  • 
globe.  It  may  therefore  be  called  the  continental  plateau. 
The  only  other  regions  of  elevation  rise  in  small,  isolated 
areas  in  various  localities,  the  largest  being  about  the  south 
pole,  and  in  the  tropical  Pacific  Ocean.  Collectively,  these 
isolated  regions  of  elevation  form  but  1070oths  of  the  land 
on  the  globe. 

The  primary  cause  of  the  elevation  of  the  conti- 
nental plateau  is  not  yet  known.  It  seems  probable  that 
the  part  of  the  earth's  crust  forming  this  region  is  lighter, 
bulk  for  bulk,  than  the  part  beneath  the  deep  sea.  This 
of  itself  would  probably  cause  the  former  to  be  a  region 
of  elevation.  As  explained  on  page  42,  the  earth's  crust 
at  a  depth  of  a  few  miles  probably  behaves  as  if  it  were 
plastic  or  liquid,  if  the  pressures  on  adjacent  portions  of  it  are 
very  unequal,  the  rock  particles  moving  or  " flowing"  side- 
ways from  under  the  region  of  greater  pressure,  until,  by 
this  transfer  of  matter,  the  weight  and  pressure  become 
uniform.  When  the  weight  thus  becomes  uniform,  the 
lines  of  equal  pressure  would  be  level.  But  to  produce 
equal  pressures,  the  lighter  part  of  the  crust  would  have 
to  be  thicker  than  the  heavier  part;  hence  its  upper  sur- 
face would  be  further  above  the  level  pressure  lines  below, 
and  would  form  a  region  of  elevation.  The  plateau  crust 
may  be  composed  of  lighter  rock  than  the  crust  of  the  sea 
bottom,  or  it  may  be  lighter  because  it  is  hotter  and  more 
expanded;  but  science  can  not  yet  satisfactorily  explain 
why  either  should  be  the  case. 

In  shape,  the  plateau  is  roughly  curved,  like  an  ir- 
regular horseshoe;  the  toe  lies  in  the  arctic  regions,  and 


REGIONS  OF  ELEVATION 


EXPLANATION. 

The  observer  Is  supposed  to  be  immediately 
above  the  north  pole,   in  the  centre  of  the 
map.      The   largest  continuous  circle 
represents  the  equator. 

Each  of  the  five   points  on  the  outside 
of  the  equator  embraces  one  fifth 
of  the  southern   hemisphere  and 
terminates   at  the   South   Pole. 


(152) 


AND    DEPRESSION. 


EXPLANATION. 

More  than  6000  feet  below   Sea   level. 
-Less       ,, 
J-Less       ,,      2000    ,,      abov 


-B=Axis  main  continental  plateau. 
-D=  ,,      Australian  branch       ,, 


154  PHYSICAL    GEOGRAPHY. 

the  two  arms  extend  into  tne  southern  hemisphere.  The 
line  AB  (pages  152,  153)  may  be  regarded  as  the  curved 
axis  of  the  main  portion  of  the  plateau.  The  deep  pocket 
formed  in  the  concavity  of  the  curve  is  the  basin  of  the 
north  Atlantic.  From  the  outer  side  of  the  main  plateau 
a  small  third  arm  extends  into  the  southern  hemisphere, 
and  separates  the  basins  of  the  Pacific  and  Indian  oceans. 
The  axis  of  this  third  arm  is  shown  by  the  dotted  line 
CD. 

Elevation  and  Coast-line. — Not  only  is  the  greater 
part  of  the  plateau  sufficiently  elevated  to  protrude  above 
the  sea  to  form  land,  but  the  highest  part,  indicated  by 
the  unshaded  portion  of  the  map,  forms  an  almost  contin- 
uous tract  along  the  outside  or  convex  margins,  while  the 
concave  margins  are  generally  low,  being  broken  only  by 
isolated  highland  regions.  The  convex  sidc3  of  the 
plateau  are  therefore  steep,  and  possess  a  very  regular 
coast-line,  while  the  concave  side  has  a  gentle  slope,  oc- 
cupying the  greater  part  of  the  width  of  the  plateau,  and 
continues  beneath  the  sea,  fringing  that  side  with  a  greater 
width  of  shallow  water.  The  coast-line  of  the  low,  con- 
cave margin  is  made  very  irregular  by  several  deep  in- 
dentations which  admit  the  sea  far  on  the  plateau  to  form 
great  continental  seas.  The  largest  of  these  are  the 
Arctic  Ocean,  the  Mexican-Caribbean  Sea,  the  Mediter- 
ranean, and  the  seas  of  the  Malay  Archipelago,  and  they 
are  located  where  the  bends  of  the  axis  are  sharpest. 

Continents. — The  depressions  occupied  by  the  Arctic 
Ocean  and  the  Malay  seas,  extend  entirely  across  the 
plateau,  breaking  through  the  high,  convex  margin  in 
Bering  Strait  in  the  one  locality,  and  in  the  several  nar- 
row straits  between  the  Sunda  Islands  (Sumatra,  Java, 
Timor,  etc.),  in  the  other  locality.  The  land  of  the 
plateau    is   thus    separated    into   three    great,    continuous 


DIVISIONS    OF   THE    LAND.  1 55 

masses,  or  continents,  and  numerous  smaller,  isolated 
masses,  or  islands.  The  three  continents  collectively  con- 
tain more  than  92%  of  the  land  on  the  globe.  The 
islands  rising  from  the  continental  plateau  are  distinguished 
as  continental  islands;  collectively,  they  comprise  almost 
7%  of  the  land  on  the  globe.  The  continents  are  very 
unequal  in  size ;  the  largest,  or  Eastern  Continent,  con- 
tains about  59%  of  all  the  land ;  the  next  in  size,  or  the 
Western  Continent,  almost  28%  ;  and  the  smallest,  or  the 
Australian  (southern)  Continent,  is  the  only  one  lying  en- 
tirely in  the  southern  hemisphere,  and  contains  less  than 
6f0  of  the  land  on  the  globe. 

TOTAL  lANn    1 I 

EASTERN  (mainland) ■^HHaflBnHMHHBBHBHIMHBl 

WESTERN         »        .mnBHOi 

AUSTRALIAN     »        Hi 

CONTINENTAL  ISLANDS ■■■ 

OCEANIC    ISLANDS 1 

UNITED    STATES, . V~J 

Fig.  66. — Relative  Areas  of  Continents  and  Islands. 

Grand  Divisions. — The  depression  of  the  Mexican- 
Caribbean  Sea  penetrates  to  the  narrow  highland  margin 
on  the  convex  side  of  the  plateau,  and  determines  the 
two  natural  and  nearly  equal  grand  divisions  of  the 
Western  Continent — North  America  and  South  America. 
A  corresponding  depression  in  the  opposite  arm  of  the 
main  plateau  is  occupied  by  the  Mediterranean,  and  con- 
tinues across  the  plateau  in  the  narrow  and  gorge-like  de- 
pression occupied  by  the  Red  Sea.  The  heads  of  these 
seas  almost  meet  at  the  narrow  Isthmus  of  Suez,  and  thus 
nearly  detach  one  third  of  the  Eastern  Continent  from  the 
rest  to  form  the  natural  grand  division — Africa.  The  re- 
mainder of  the  Eastern  Continent  strictly  forms  a  single 
natural  grand  division — Euro-Asia. 


156 


PHYSICAL    GEOGRAPHY. 


Before  the  extent  of  the  Black  and  Caspian  seas  was  accurately 
known,  their  depressions  were  supposed  to  divide  this  grand  division 
into  two  parts,  which  were  named  Asia  and  Europe.  The  error  was 
subsequently  discovered,  but  the  names  remained,  and  the  Eastern 
Continent  is  still  said  to  be  composed  of  three  grand  divisions, 
though  Europe  occupies  but  little  more  than  one  tenth  of  its  area,  and 
the  boundary  between  Europe  and  Asia  is  arbitrary  rather  than  real. 
Each  of  these  five  grand  divisions  is  frequently  though  wrongly 
called  a  continent. 

Distribution  of  Continental  Islands. — More  than 
85%  of  the  area  of  continental  islands  occurs  in  the  great 


LAND 


Fig.  67.— Continental  Plateau  between  Asia  and  Australia. 

bends  of  the  continental  plateau ;  thus,  almost  one  half 
(46^)  occurs  in  the  Arctic  Ocean,  and  by  far  the  largest 
part  of  this  island  area,  including  Greenland,  Iceland,  and 
Great  Britain,  occurs  on  the  concave  margin,  or  rim,  of  the 
plateau  (see  chart,  pages  152,  153).  More  than  one  third 
(36%)  of  the  continental  island  area  occurs  in  the  great 
bend  of  the  Australian  arm  of  the  plateau,  where  it  forms 
the  Malay  Archipelago  (Fig.  6f)  and  the  continuous  chain 
of  islands  along  the  concave  margin  of  the  bend,  of  which 


DIVISIONS    OF   THE    LAND.  1 57 

Japan,  the  Philippines,  and  New  Guinea  are  the  principal 
groups.  More  than  3^  of  the  continental  island  area 
occurs  in  the  minor  bends  of  the  plateau  occupied  by  the 
Caribbean  and  Mediterranean  seas,  the  islands  in  the 
former  locality  occurring  along  the  concave  rim  of  the 
bend  as  the  chains  of  the  West  Indies  (see  chart,  pages 
152,    153). 

The  remaining  15$,  of  the  continental  island  area  embraces 
islands  which  occur  along  the  margins,  but  are  not  confined  to  the 
concave  margin  of  the  plateau.  About  two  fifths  of  this  area  com- 
pose islands  lying  close  to  the  continents,  and  well  within  the 
limits  of  the  plateau,  as  Newfoundland,  Tasmania,  and  Ceylon,  and 
the  Alaskan  and  Chilean  islands.  The  remaining  three  fifths  com- 
pose the  two  groups  of  lcrge  islands — Madagascar  and  New  Zea- 
land. These  are  somewhat  exceptional  among  continental  islands, 
because  they  occupy  outlying  spurs,  almost  if  not  quite  detached 
from  the  continental  plateau,  and  because  many  of  the  forms  of  life 
on  these  islands  differ  from  those  of  the  adjacent  continent.  These 
islands  are  properly  classed  as  continental  islands,  however,  since 
their  geological  structure  and  some  of  their  forms  of  life  correspond 
to  those  of  the  adjacent  continent,  and  because  the  water  which  sep- 
arates them  from  the  continent  is  shallow  in  comparison  with  that  on 
the  opposite  or  oceanic  side  of  the  islands. 

Oceanic  Islands. — About  y^Q-ths  of  the  land  on  the 
globe  occurs  in  numerous  very  small  masses  in  the  midst 
of  the  oceans  and  far  from  the  continents.  They  occur  in 
each  of  the  three  great  oceans,  but  are  most  numerous  in 
the  tropical  Pacific,  where  they  lie  in  long,  nearly  straight, 
or  gently  curving  lines  extending  in  a  general  north-west 
and  south-east  direction.  They  contain  none  of  the  kinds 
of  rock  which  compose  the  greater  part  of  the  great  land 
masses,  and,  unlike  all  the  continents  and  continental 
islands,  they  contain  no  native  four-footed  animals. 

These  islands  are  thought  to  be  the  tops  of  volcanic  cones  which 
have  built  themselves  up  from  great  depths  by  the  solidification  of 
successive  outflows  of  melted  rock  or  lava  around  some  aperture  in 
the  earth's  crust.     They  generally  rise  from  the  crest  of  the  low  sub- 


«58 


PHYSICAL    GEOGRAPHY. 


Fig.  68.— Coral  Formations. 

marine  ridges  or  plateaus  which  traverse  the  ocean  basins,  which 
accounts  for  the  lineal  arrangement  of  the  oceanic  island  groups. 
The  submarine  ridges  are  probably  formed  in  the  same  general 
manner  as  the  continental  plateau, — by  differences  in  the  temperature 
and  density  of  adjacent  regions  of  the  earth's  crust.  These  differ- 
ences, however,  are  probably  relatively  slight,  hence  the  submarine 
ridges  do  not  stand  so  high  as  the  continental  plateau.  Being  larger, 
the  Pacific  contains  a  greater  number  of  ridges  than  other  oceans. 
The  outflows  of  lava  which  largely  compose  oceanic  islands,  are 


DIVISIONS    OF   THE   LAND.  1 59 

probably  the  direct  result  of  the  fracturings  of  the  earth's  crust  and 
the  heat  generated  by  these  movements  of  upheaval. 

Coral  Islands  and  Reefs. — In  the  shallow  water  about 
the  shores  of  many  oceanic  islands,  and  in  fact  of  all 
coasts  where  the  water  is  warm  and  clear,  low,  rocky  reefs 
frequently  occur.  These  rise  to  about  the  level  of  low 
tide,    and  are  composed  of  the  peculiar  coral  limestone. 

Some  oceanic  islands  rising  from  great  depths  seem  to 
be  composed  entirely  of  this  limestone.  Such  islands 
never  rise  more  than  10  or  12  feet  above  sea-level,  and 
usually  take  the  form  of  a  narrow  strip  or  ring  of  rocky 
land,  wholly  or  partially  surrounding  a  shallow  lake,  or 
lagoon,  of  sea-water.  These  islands  are  called  atolls,  and 
are  common  in  the  warm  parts  of  the  Pacific  and  Indian 
oceans.  Although  apparently  composed  entirely  of  coral 
rock,  it  is  probable  that  this  rock  merely  covers  and  con- 
ceals a  volcanic  foundation  at  a  comparatively  slight  depth. 

Coral  reefs  and  islands  are  composed  of  rock  which  is  nearly 
pure  carbonate  of  lime,  and  is  remarkable  in  its  manner  of  forma- 
tion. Myriads  of  sea  animals,  called  polyps,  live  in 
vast  colonies  on  the  bottom  of  clear,  shallow,  tropical 
seas.  The  skeletons  of  these  animals  are  carbonate 
of  lime  extracted  by  the  polyps  from  the  sea-water. 
The  general  cross  section  of  a  polyp  is  shown  in 
Fig.  69,  the  black  portion  indicating  the  stony  skele- 
ton. As  the  polyps  grow  upward,  the  lower  part  of 
their  cylindrical  skeleton  becomes  a  solid  stalk  or  stem  of  stone, 
from  the  sides  of  which  other  polyps  grow  outward,  thus  eventually 
forming  an  intricate  network  of  stone  branches.  The  surfaces  of  both 
this  network  and  the  parent  stem  may  be  covered  with  living  polyps. 
Branches  are  constantly  being  broken  off  and  ground  into  sand  by 
the  force  of  the  waves,  and  this  sand  slowly  fills  up  the  spaces  be- 
tween the  various  stems  and  branches  until  the  whole  becomes 
cemented  into  solid  coral  reef  rock.  This  also  gradually  grows  by  the 
same  process,  both  upward  to  the  surface  of  the  water,  and  outward 
to  a  depth  of  about  20  fathoms,  beyond  which  polyps  on  its  surface 
can  not  live.     If  the  water  and  bottom  close  to  the  shore  are  clean, 


160  PHYSICAL    GEOGRAPHY. 

the  reef  extends  quite  to  the  shore,  and  is  called  a  fringing  reef; 
but  if  the  water  and  bottom  are  muddy,  a  channel  of  water  inter- 
venes between  the  shore  and  the  reef,  and  the  latter  is  then  called  a 
barrier  reef  Polyps  thrive  best  in  a  heavy  surf;  hence,  the  outside  of 
a  barrier  reef  grows  faster  than  the  inside,  which  contains  but  few 
live  polyps,  and  often  does  not  grow  as  fast  as  it  is  dissolved  away 
by  the  sea-water.  Many  islands  of  the  Pacific  are  almost  sur- 
rounded by  a  barrier  reef,  separated  from  the  shore  by  a  broad 
channel  of  water  several  fathoms  deep.  Atolls  are  much  like  such 
barrier  reefs,  except  that  they  inclose  no  island.  Since  polyps 
thrive  only  to  a  depth  of  20  fathoms,  an  atoll  can  be  started  only  in 
shallow  water.  Rising  as  a  reef  to  the  surface  in  such  a  shallow 
place  in  the  open  ocean,  it  naturally  assumes  the  irregular  circular 
shape  around  a  shallow  lagoon,  for  the  heavy  surf  favors  the  rapid 
growth  of  the  outside  edge,  while  the  interior  gradually  dissolves 
away  under  the  action  of  the  sea-water.  The  size  of  the  inclosed 
lagoon  thus  very  gradually  increases  by  the  seaward  growth  of  the 
encircling  reef.  Pieces  broken  from  the  outer  edge  of  the  reef  and 
cast  up  by  the  waves  gradually  raise  the  reef  above  the  surface  of 
high  tide,  while  wind  and  currents  bring  seeds  which  take  root  and 
cover  the  atoll  with  vegetation.  Larger  pieces,  broken  off  by  the 
waves,  fall  to  the  bottom  and  form  a  talus,  or  slope,  of  fragments  of 
coral  rock,  on  which  the  living  surface  portion  of  the  atoll  slowly 
advances  into  deep  water. 

Antarctic  Lands. — In  addition  to  the  known  land,  an 
indefinite,  but  probably  a  comparatively  small  area  of 
land  is  supposed  to  occur  within  the  antarctic  circle. 
Whether  this  land  area  is  continuous,  or  whether  it  is 
broken  up  into  an  island  group,  is  not  known ;  but  as  the 
rocks  found  on  the  bottom  of  the  southern  oceans,  and 
which  have  evidently  been  dropped  by  antarctic  icebergs, 
resemble  the  rocks  of  the  known  continents  and  conti- 
nental islands,  it  is  inferred  that  the  antarctic  lands  should 
be  classed  with  them  rather  than  with  the  oceanic  islands. 


CHAPTER  XII. 

THE    SURFACE    OF   THE    LAND. 
Go  up  and  view  the  country. — Joshua  vii  :  2. 

Average  Elevation. — The  average  elevation  of  the 
land  on  the  globe  is  about  2,000  feet  above  sea-level. 
There  is  of  course  land  much  higher  than  this  in  each 
grand  division ;  but  if  the  entire  land  surface  were  reduced 
or  increased  to  a  uniform  elevation,  the  resulting  level  sur- 
face would  be  about  2,000  feet  above  the  sea. 


Grand  Division. 

Average  Elevation. 

Highest  Elevation. 

Asia 

2,884  feet. 

Mount  Everest,  29,002  feet. 

Africa 

1,975     " 

Kilimanjaro,        20,065    " 

North  America 

1.954    " 

St.  Elias  Alps,     19,500  (?) 

South  America 

1.764     " 

Aconcagua,         23,910     " 

Australia 

1,189     " 

Clarke,                  7,256     " 

Europe 

958     " 

Elbrooz,               18,493     " 

Lowland  and  Highland. — Hence,  in  comparison  with 
the  land  surface  of  the  globe,  any  land  whose  surface  lies 
at  a  less  elevation  than  2,000  feet  may  be  considered  as 
lowland,  while  all  land  at  a  greater  elevation  may  be  re- 
garded as  highland. 

The  Surface  of  both  highland  and  lowland  is  uneven. 
It  does  not  slope  uniformly  either  in  rate  or  in  direction, 

(161) 


1 62  PHYSICAL  GEOGRAPHY. 

over  any  considerable  area.  In  consequence  of  the  diver- 
sity of  slope,  the  surface  of  both  lowlands  and  highlands 
is  composed  of  a  series  of  relatively  high  regions,  sepa- 
rated from  each  other  by  a  series  of  relatively  low  regions. 
These  regions  are  of  course  high  and  low  only  in  compar- 
ison with  one  another,  for  the  low  regions  of  the  high- 
lands have  a  greater  elevation  above  the  sea  than  the  high 
regions  of  the  lowlands. 

Mountains  and  Hills. — A  region  is  usually  called  a 
mountain  in  which  the  elevation  of  the  surface  changes 
about  1,000  feet  or  more  by  a  slope  rapid  enough  to  be 
plainly  perceptible  to  the  eye.  If  the  slope  be  perceptible, 
but  the  change  of  elevation  be  much  less  than  1,000  feet, 
the  region  is  called  a  hill.  A  relatively  high  point  from 
which  the  surface  slopes  perceptibly  in  all  directions,  is 
called  a  peak.  A  long  but  very  narrow  region  from  which 
the  surface  slopes  downward  mainly  in  two  opposite  direc- 
tions, is  called  a  ridge  of  mountain  or  hill.  By  far  the 
greater  number  of  mountains  in  the  world  occur  in  the  form 
of  ridges,  or  of  ranges  or  chains ;  that  is,  a  succession  of 
closely  adjacent  ridges,  whose  lengths  lie  along  the  same 
general  course.  A  relatively  high  region,  composed  of 
two  or  more  roughly  parallel  mountain  chains,  separated 
by  elevated  land,   constitutes  a  mountain  system. 

The  dividing  line  between  mountains  and  hills,  based  upon  alti- 
tude alone,  is  purely  arbitrary.  Eminences  called  mountains  in 
flat  regions,  would  be  called  hills  in  regions  where  much  higher 
eminences  occur.  A  better  plan  would  be  to  confine  the  term 
"hill"  to  relatively  low  eminences,  composed  of  rock  arranged  in 
nearly  horizontal  layers. 

Plateaus  and  Plains  are  extensive  regions  having  a 
comparatively  flat  surface,  or  one  whose  general  slope  is  so 
gradual  as  to  be  scarcely  perceptible.  Such  regions  are 
generally  called  plains  in  lowlands,  and  plateaus  in  high- 
lands;   but  where    the    lowland   rises   imperceptibly   into 


THE  SURFACE  OF  THE  LAND.         1 63 

highland,  the  apparently  flat  or  gently  undulating  surface 
is  called  a  plain  in  both  regions. 

This  is  the  case  with  the  Great  Plains  east  of  the  Rocky  Mount- 
ains, which  slope  imperceptibly  downward  to  the  east  from  an  eleva- 
tion of  about  6,000  feet.  On  the  other  hand,  relatively  high,  flat 
regions  of  the  lowlands,  when  separated  by  steep  slopes  from  lower 
regions,  are  frequently  called  plateaus  ;  thus,  the  greater  part  of  the 
Cumberland  and  Appalachian  plateaus  lies  at  a  less  elevation  than 
2,000  feet  above  the  sea. 

Valleys  are  usually  understood  to  be  long,  V-shaped 
depressions,  whose  side  slopes  are  very  perceptibly  steep, 
and  whose  bottoms  have  a  much  more  gradual  slope  in  the 
direction  of  the  valley's  length.  Valleys  occur  in  every 
region  of  the  land,  but  are  more  numerous,  deeper,  and 
all  their  slopes  are  steeper  in  highland  than  in  lowland 
regions,  and  among  hills,  mountains,  and  plateaus  than 
on  plains.  Indeed,  it  is  the  great  number  of  very  deep 
and  steep  valleys  which  give  to  mountain  regions  their 
very  rough  and  uneven  contour. 

The  term  valley  is  frequently  used  in  a  broader  sense  to  include 
all  the  relatively  low  region  lying  between  contiguous  regions  of 
highland.  Thus,  most  of  the  United  States  between  the  Rocky  and 
Appalachian  mountains  is  said  to  lie  in  the  Mississippi  Valley.  In 
this  case  the  general  slope  of  the  sides  of  the  valley  is  impercepti- 
ble, and  is  broken  by  steeper  minor  slopes  into  mountains,  hills, 
plateaus,  plains,  and  smaller  valleys. 

Steepness  of  Slopes. — All  plainly  perceptible  slopes 
are  generally  supposed  to  be  much  steeper  than  they 
really  are,  while  imperceptibly  sloping  surfaces  of  course 
seem  level.  The  Great  Plains  east  of  the  Rocky  Mount- 
ains have  an  average  slope  of  about  seven  feet  to  the 
mile.  This  is  entirely  imperceptible.  Probably  an  incli- 
nation of  between  200  and  300  feet  to  the  mile  is  required 
before  any  slope  can  be  detected  in  the  absence  of  a  level 
surface  with  which  to  compare  it;  such  a  slope  makes  an 
angle  of  about  30  with  the  horizontal.     The  great  majority 


164 


PHYSICAL    GEOGRAPHY. 


Slopes.       275  ft.  to  the  mile 


3000  ft.  to  the  mile     4500  ft.  to  the  mile. 
Fig.  70. 


of  steep  slopes  make  an  angle  of  less  than  300  with  the 
horizontal;  that  is,  they  rise  at  a  rate  of  less  than  3,000 
feet  to  the  mile,  while  slopes  of  400  (4,500  feet  to  the 
mile)  occur  only  in  naked  rock,  and  except  when  very 
short  are  exceedingly  rare.  Actual  vertical  precipices  are 
never  very  high,  for  not  only  does  a  slope,  or  talus,  tend 
to  form  against  the  bottom  of  the  cliff  by  the  accumula- 
tion of  fragments  detached  from  the  top  by  the  weather 
(Fig.  71),  but  the  enormous  weight  of  the  overlying  strata 
would  crush  the  rocks  forming  the  bottom  of  a  very  high 
cliff,  and  cause  them  to  "  creep"  outward,  thus  reducing 
the  lower  vertical  part  of  the  cliff  to  a  steep  slope. 


Fig.  71.— A  Line  of  Cliffs,  with  Talus  (Red  Gate,  Utah). 

Some  of  the  steepest  general  slopes  in  the  United  States  are  shown 
in  the  diagrams  opposite.  In  nature  they  are  broken  by  minor  irreg- 
ularities which  jender  them  for  very  short  distances  alternately  steeper 
and  flatter  than  represented,  but  the  diagrams  show  the  average 
or  general  slopes,  and  the  height  of  these  is  seen  to  be  in  general 


w  ROCKY      MOUNTAINS, 


Horizontal  and  Vertical  scales  the  same. 

GREEN      MOU  NTAINS  , 
vN.W  Mt.Greyloch 


M ASSACH  USETTS 


W.            Appalachian 
00 Plateag 


ALLEGHANY    MOUNTAINS,  WEST    VIRGINIA. 

Alleghany  Mts.  E. 


COLORADO. 


(—  5000-FTV-ABOVE-8EA- 
0  2 


WASATCH      MOUNTAINS, 
Timpanogos   Peak 


MOUNT      SHASTA, 


16000— S 


CALIFORNIA. 
ML  Shasta 


GRAND    CANON    (near  toroweap),  ARIZONA. 

Colorado         ,„„„  _,.  Kanab 

-Plateau 


JOOO-FTt-ABOVE-ISEA 


Profiles  of  Steep  Slope*. 


(165) 


1 66  PHYSICAL  GEOGRAPHY. 

much  less  than  the  length.  One  general  law  is  well  illustrated  by 
these  diagrams  :  almost  all  slopes  of  the  land  surface  gradually  become 
flatter  as  they  are  descended.  The  reason  for  this  will  be  explained 
later  (page  220).     The  steepest  long  slope  shown  in  the  diagrams 

2000  f. is  that  of  the   sides  of   the  Grand 

"T  '         :  Canon  of  the  Colorado  River,  where 

^  uj  the  surface  falls  about  3,000  feet  in 

^^_  I.      -  less  than  half  a  mile.    This  Cation  is 

^3^  0  often  described  as  having  nearly  per- 

_)         §      Q  pendicular  sides.     This    is   not    the 

^v.    I      &  ,      case,  as  is  shown  in  the  enlarged  dia- 

Taj      -j        £r      gram  (Fig.  72),  which  shows  the  pro- 

3J,      o      §        file  of  one  side  of  the  inner  gorge  of 

^^^^r  this  canon  near  Toroweap,  where  the 

slope,  though  not  quite   so  long,  is 

about  as  steep  as  at  any  other  point.    (P.  223.) 

Highlands  and  Lowlands  of  North  America. — North 
America  contains  two  great  mountain  systems:  the  Appa- 
lachian system  in  the  east,  and  the  Cordillera  system  in 
the  west.  Each  system  is  composed  of  numerous  ranges 
or  ridges,  roughly  parallel  with  each  other  and  with  the 
respective  coasts  of  the  grand  division.  The  Cordillera  is 
much  the  larger  system  in  every  way.  It  is  bordered  by 
two  great  chains,  the  Rocky  Mountains  on  the  east,  and 
the  Cascade  Mountains,  the  Sierra  Nevada,  and  the  Sierra 
Madre  of  Mexico  on  the  west.  Between  these  are  many 
isolated  ranges.  In  each  of  these  chains  are  many  peaks 
between  12,000  and  15,000  feet  high,  while  near  Mt.  St. 
Elias  in  the  north  are  peaks  over  19,000  feet  high.  In 
the  south  Orizaba  rises  over  18,000  feet  above  sea- 
level.  These  chains  and  the  numerous  shorter  ranges 
and  ridges  between  them  rise  from  a  rough  plateau  which 
maintains  a  general  elevation  of  over  6,000  feet  east  of 
the  Wasatch  Mountains  of  Utah,  but  of  less  than  5,000 
feet  to  the  west  of  it.  This  relatively  low  portion  of  the 
plateau  extending  west  and  south-west  from  Great  Salt 
Lake  to  the  Sierra  Nevada,    is  called  the   Great  Basin. 


Ut>?) 


1 68 


PHYSICAL    GEOGRAPHY. 


Toward  the  northern  and  southern  extremities  of  the  sys- 
tem active  volcanoes  occur,  while  in  the  western  part  of 
the  central  portion  numerous  volcanic  cones  and  other 
evidences  of  recent  volcanic  action  are  found. 

The  Appalachian  system  throughout  the  southern 
portion  of  its  extent  consists  of  many  sharp,  parallel 
ranges  or  ridges  rising  from  lowland  elevations  of  less  than 
i, OCX)  feet  to  a  general  elevation  of  between  2,000  and 
3,000  feet  above  the  sea.     The  general  elevation  of  the 


B-WILES-^ 


S.       c 

•§ 

ON     PARALLEL    OF    40       LATITUDE 
(Heights  exaggerated  100  times) 

.§!  Height 

N. 

2 

N.E. 

Gulf   of     f 

MISSISSIPPI      VAL. 

■2  §-o/  Ld\nd 

£8 

Hudson 

Bay 

1  •  a 

Mexico       0. 

I 

— S—i — 

M 

'-v- y-'^~~ 

MILES 

10 

00                               1600 

20 
Mil 

o"o— 

ES 

_ 

" 

ON     MERIDIAN     OF    90  "  W.     LONGITUDE 
Fig.  73-—  Two  Sections  Across  North  America. 


eastern  range  is  greater  than  that  of  the  western  ranges. 
Its  highest  peaks  are  Black  Dome  (6,700  feet)  in  North 
Carolina,  and  Mt.  Washington  (6, 200  feet)  in  New  Hamp- 
shire. From  the  summit  of  the  western  range  the  Appa- 
lachian "  plateau,"  with  an  elevation  of  a  little  less  than 
2,000  feet,  slopes  westward,  merging  imperceptibly  into 
the  Mississippi  Valley. 

The  portion  of  the  Appalachian  system  lying  north  of  the  St. 
Lawrence  River  is  called  the  Laurentide  Mountains,  and  is  very 
different  from  the  southern  portion  of  the  system.  It  is  virtually  a 
low  plateau  having  an  elevation  of  about  2,000  feet,  from  which  rise 
occasional  more  or  less  isolated  peaks  or  short  ridges,  which  are 
worn  to  a  smooth  and  rounded  outline  ;  the  height  of  these  peaks  is 


THE  SURFACE  OF  THE  LAND.         1 69 

generally  less  than  3,000  feet  above  the  sea,  though  the  highest  is 
thought  to  exceed  this  elevation.  Both  the  Appalachian  and  Lauren- 
tide  mountains  contain  many  evidences  of  very  ancient,  but  none  of 
recent  volcanic  action. 

The  Lowlands  of  North  America  lie  chiefly  between 
the  two  mountain  systems,  and  extend  from  the  Gulf  of 
Mexico  to  Hudson  Bay  and  the  Arctic  Ocean.  Although 
broken  by  short  slopes  into  valleys,  local  undulations, 
and  hills,  which,  in  the  case  of  the  Ozarks  of  Missouri, 
and  the  Wichitas  of  Oklahoma,  are  dignified  by  the 
name  "mountains,"  still  the  general  slope  is  entirely 
imperceptible,  and  rises  from  both  north  and  south  to  a 
maximum  elevation  of  about  1,800  feet  in  the  Height  of 
Land  north  of  the  Great  Lakes. 

South  America  contains  three  mountain  systems:  the 
Cordillera  of  the  Andes,  extending  along  the  whole  west 
coast,  the  Brazilian  system  in  the  east,  and  the  Pacaraima 
system  in  the  north. 

The  Cordillera  of  the  Andes,  though  much  narrower, 
is  almost  twice  as  high  as  the  Cordillera  system  of  North 
America.  It  consists  in  general  of  two  main  chains 
roughly  parallel  with  each  other  and  with  the  west  coast, 
from  which  the  surface  rises  by  a  very  steep  slope,  the 
crest  of  the  westerly  chain  lying  in  some  places  not  more 
than  65  miles  from  the  sea-shore,  and  bearing  many  peaks 
between  16,000  and  20,000  feet  in  elevation.  Aconcagua, 
the  highest  peak,  rises  almost  24,000  feet  above  the  sea. 
The  eastern  chain  is  not  quite  so  high  as  the  western; 
but  near  the  central  portion,  where  both  chains  are  high- 
est, it  has  several  peaks  of  20,000  feet  and  more.  This 
system  contains  throughout  its  length  many  active  or  re- 
cently extinct  volcanoes. 

Between  the  central  portion  of  the  chains  is  a  very  high,  broken 
plateau,  whose  elevation  ranges  from  12,000  to  14,000  feet.     In  the 
north,  the  eastern  chain  bears  gradually  off  to  the  east  parallel  with 
P.  G.-xo. 


170 


PHYSICAL    GEOGRAPHY. 


{Heights  exaggerated   100  times) 


Brazilian    Highland      a 


AM  AZ 


ON      VAL 


LEY 


Atlantic 


JUST      SOUTH      OF      EQUATOR 


g  2-2-miles-St 


Sf- 


5?c3- 


-NvSS 


PARAGUAY   VALLEY 


XAJl 


ORI  <OCO   VAL.      $q 


BUENOS    AYRES    TO     MOUTH    OF    ORINOCO 
Fig.  74.  -Two  Sections  across  South  America. 

the  Caribbean  sea-coast,  while  the  western  chain,  decreasing  in  alti- 
tude, bears  to  the  north-west  to  form  the  highland  connection, 
through  the  Isthmus  of  Panama, with  the  Cordillera  system  of  North 
America. 

The  Brazilian  and  Pacaraima  systems  are  much 
lower  and  less  continuous  than  the  Andes.  The  highest 
peaks  are  about  9,000  and  8,000  feet  respectively,  though 
peaks  higher  than  6,000  feet  are  extremely  rare.  The 
Brazilian  system  is  composed  of  three,  and  the  Pacaraima 
of  two,  chains  roughly  parallel  with  each  other  and  with 
the  nearest  coast.  The  Brazilian  system  rises  rather 
abruptly  from  a  low  and  narrow  coast  plain,  but  the  Paca- 
raima system  and  the  landward  side  of  the  Brazilian 
system  have  their  base  on  a  highland  which  attains  its 
elevation  of  about  2,000  feet  by  an  entirely  imperceptible 
ascent.  The  eastern  highlands  of  South  America,  like 
those  of  North  America,  contain  no  vestiges  of  recent  vol- 
canic activity,  but  indications  of  ancient  volcanism  are 
found. 

The  Lowlands  of  the  greater  part  of  South  America 
are  exceedingly  flat.  The  Amazon  River,  where  it  leaves 
the  Andes,  2,000  miles  from  its  mouth,  is  only  500  feet 
above  the  sea.  In  eastern  Bolivia,  where  the  western 
highland  approaches  the  Brazilian  highland  most  closely. 


THE  SURFACE  OF  THE  LAND.         171 

the  lowland  between  them  is  scarcely  1,000  feet  above 
the  sea,  while  west  of  the  Pacaraima  system  the  lowland 
has  but  half  this  elevation. 

Euro-Asia,  the  largest  grand  division  of  the  land,  and 
the  most  irregular  in  shape,  has  the  most  extensive,  the 
highest,  and  the  most  irregular  mountain  system,  as  well 
as  the  most  extensive  lowlands. 

Mountains  and  Plateaus. — The  highland  region  ex- 
tends along  the  entire  southern  portion  of  the  grand  divi- 
sion from  Portugal  to  Bering  Strait,  a  distance  of  10,000 
miles.  Though  cut  down  to  sea-level  in  one  place — the 
outlet  to  Black  Sea — this  highland  region,  with  its  various 
plateaus  and  mountain  ranges,  may  be  regarded  as  a 
single  vast  mountain  system.  The  general  width  and 
height  of  the  system  increase  from  either  extremity 
toward  the  center,  where  the  highland  region  is  2,500 
miles  broad,  from  north-west  to  south-east.  The  plateaus 
at  either  extremity  of  the  system  have  an  elevation  of 
about  2,500  feet,  which  gradually  increases  to  about  5,000 
feet  near  the  central  region,  where  the  surface  abruptly 
rises  to  form  the  extensive  Pamir-Thibet  plateau,  at  an 
elevation  of  between  12,000  and  15,000  feet.  This  high 
plateau  has  a  length  of  about  2,000  miles  and  an  average 
width  of  450  miles,  an  area  equal  to  that  of  the  United 
States  east  of  the  Mississippi.  The  broad  plateaus  of 
Asia  are  generally  lower  in  the  center  than  at  the  mar- 
gins. Thus,  the  central  parts  of  the  Persian,  East  Tur- 
kistan,  and  Mongolian  plateaus  are  1,200,  2,200,  and  3,000 
feet  respectively,  while  at  the  base  of  the  surrounding 
mountains  their  elevation  is  about  5,000  feet. 

Though  when  distant  mountain  ranges  of  Euro-Asia  are  com- 
pared, they  vary  greatly  in  the  direction  of  their  trend,  yet  when  the 
system  is  viewed  as  a  whole  the  various  chains  are  seen  to  be 
roughly  parallel  with  the  axis  of  the  highland  region,  with  the  south- 


172  PHYSICAL    GEOGRAPHY. 

ern  or  eastern  coast  of  the  grand  division,  and  with  adjacent  chains. 
Where  the  highlands  are  broad,  the  mountains  generally  rise  from 
their  northern  and  southern  margins,  inclosing  the  plateaus  between. 
Several  active  volcanoes  occur  in  this  great  mountain  system,  and 
many  signs  of  recent  volcanic  action  are  found  throughout  its 
extent  from  Spain  to  Kamchatka.  The  gradual  increase  in  the 
altitude  of  the  mountains  toward  the  central  region  of  the  high- 
lands is  as  follows : 


Spain,                      Long., 

50  W.  to  o°. 

Peaks, 

n,oooto  12,000  feet. 

Switzerland, 

' 

8°  E.  to 

15° 

E. 

** 

13,000  to  16,000   " 

Caucasia  and  Persia, 

' 

420  E.  to 

5o° 

E. 

" 

17,000  to  19,000    " 

Pamir  and  Thibet, 

' 

700  E.  to 

900 

E. 

«' 

22,000  to  29,000   " 

Thian  Shan, 

" 

8o°E. 

<< 

21,000  feet. 

Khin  Gan, 

« 

1050  E. 

" 

11,500    " 

<«         << 

' 

1170  E. 

" 

9,000    " 

Stanovoi  Mountains, 

' 

1350  E. 

<< 

4,000    " 

Three  wholly  or  partially  detached  regions  of  highland  lie  south 
or  east  of  the  main  mass,  being  separated  from  it,  however,  by  low- 
lands of  small  extent  in  comparison  with  the  great  lowland  region 
to  the  north.  These  highlands  constitute  the  plateaus  of  Arabia, 
India,  and  the  coast  region  north  of  Corea.  They  have  an  average 
altitude  of  less  than  2,500  feet,  and  are  bordered  by  mountains  of 
very  moderate  elevation.  A  mountain  range,  partially  submerged, 
and  roughly  parallel  with  the  east  coast,  forms  the  peninsula  of 
Kamchatka  and  the  chain  of  islands  of  which  Japan,  Formosa,  the 
Philippines,  and  New  Guinea  are  the  largest.  Through  Borneo  and 
Celebes  this  chain  is  connected  with  another  more  continuous  chain, 
which,  diverging  from  the  east  end  of  the  Thibet  plateau,  forms  the 
Malay  peninsula  and  the  islands  of  Sumatra,  Java,  and  Timor. 
Most  of  these  islands  contain  peaks  of  from  9,000  to  13,000  feet 
high,  and  very  many  of  these  are  active  volcanoes. 

Lowlands. — The  northern  part  of  the  grand  division  is 
a  vast  region  of  continuous  lowland,  having  an  extreme 
length  of  10,000  miles,  and  a  greatest  width,  in  the  longi- 
tude of  the  Caspian  Sea,  of  about  2,500  miles.  The 
lowest  part  of  this  region  is  covered  by  the  Caspian  Sea  to 
a  depth  of  3,600  feet,  but  the  surface  of  the  sea  is  still  85 
feet  below  sea-level.     From  this   depression   the   surface 


(173) 


174 


PHYSICAL    GEOGRAPHY. 


S.N.W. 


2--' 
S. 


500  MILES       1000  1500  2000 

ARABIAN     SEA    NORTHWEST    TO     LOFODEN     IS. 
(All  uertioal  distances  exaggerated  100  times) 


6-MILE 

3 Him 

alayas 

I 

A.       S 

n 

Kuen 

Lun                 tyian    _- 

ll 

4-J 

8 

5 

f\f\  Thibet    " 

Turkistan    *f 

nan    » 

r             B 

S    I    B    E 

R    I    A 

* 

42 

3 

i 

r'\ZZy\ 

5   (\ 

(1  A  A? 

o 

2  -2 

&J  I 

MS 

o 

I   1 

o 

-W 

l/  L^-^. 

^ 

) 

~500~MILES                1000                               1500                              2000                               2500                             3000              r=5r 

BAY    OF    BENGAL    TO    KARA    SEA 

Fi&-  75»— Two  Sections  across  Euro-Asia. 

rises  to  the  north  by  imperceptible  slopes  to  an  elevation 
of  about  1,000  feet,  whence  it  imperceptibly  descends  to 
the  north  coast.  These  plains  are  called  steppes  in  the 
south,  and  tundras  in  the  north.  This  vast  region  rises  in 
two  places  only,  and  then  by  almost  imperceptible  slopes, 
above  the  limit  of  lowlands  (2,000  feet):  (1)  In  the  ex- 
treme north-west,  to  form  the  Scandinavian  plateau,  which 
falls  off  abruptly  toward  the  sea  from  a  general  elevation 
of  2,000  to  4,000  feet,  and  bears  some  points  over  8,000 
feet  high ;  and  (2)  between  Europe  and  Asia  to  form  the 
low  Ural  Mountains,  whose  highest  points  are  about  5,500 
feet.  In  the  extreme  east  the  lowlands  are  broken  by  several 
ranges  of  hills  or  low  mountains  putting  out  from  the  great 
system  to  the  south.  In  both  of  these  isolated  highlands, 
vestiges  of  very  ancient  volcanic  action  are  found. 

Surface  of  Africa. — The  main  highland  region  extends 
along  the  eastern  coast  from  the  outlet  of  the  Red  Sea  to 
the  Cape  of  Good  Hope.  This  highland  increases  in 
general  width  from  north  to  south,  and  almost  completely 
covers  the  southern  portion  of  the  grand  division. 

Three  long  tongues  of  highland  extend  to  the  north- 
west from  the  main  mass  until  they  gradually  merge  into 
the    lowland.     These    tongues   are    separated    from    each 


(*75> 


I76  PHYSICAL    GEOGRAPHY. 

other  by  two  broad  lowland  valleys  extending  southward 
from  the  great  lowland  region  which  occupies  northern 
Africa.  One  of  these  valleys  is  occupied  by  the  Nile 
River,  while  the  other  contains  the  upper  course  of  the 
Kongo,  Lake  Chad  and  its  principal  tributaries,  and  the 
upper  course  of  the  Niger.  Minor  elevations  in  the 
second  valley  have  caused  the  lower  course  of  the  Kongo 
and  Niger  to  bend  at  right  angles  with  their  upper  course, 
and  to  cut  narrow  channels  to  the  sea  through  the  souths 
western  highland  tongue.  A  small,  detached  mass  of 
highland  in  the  extreme  north-west  extends  parallel  with 
the  Mediterranean  and  Atlantic  coast,  and  forms  a  contin- 
uation  of  the  Italian  region  of  elevation,  which  curves 
sharply  back  to  westward  through  Sicily  and  the  shallow 
extension  from  that  island  to  Tunis  (see  page  152). 

It  will  be  observed  that  the  main  highland  mass  lies  roughly 
parallel  with  the  east  coast,  while  the  three  tongues  are  roughly 
parallel  with  each  other  and  with  the  north-east  and  south-west 
coasts  of  the  grand  division.  Several  active  volcanoes  and  many- 
indications  of  recent  volcanic  action  are  found  in  the  eastern  high- 
land, while  in  the  west  but  one  active  volcano  occurs  on  the  main- 
land, though  evidences  of  ancient  volcanism  are  numerous. 

Heights. — The  greatest  heights  of  the  grand  division 
rise  as  more  or  less  continuous  mountain  ranges  from  the 
margins  of  the  highlands,  and  thus  inclose  a  plateau  whose 
general  elevation  is  something  less  than  5,000  feet.  The 
greatest  heights  occur  along  the  eastern  margin,  peaks  ris- 
ing to  nearly  15,000  feet  in  Abyssinia,  and  to  20,000 
feet  near  the  equator,  where  Kilimanjaro,  the  highest 
point  in  Africa,  occurs.  The  peaks  near  the  Zambezi 
sink  to  7,000  or  8,000  feet,  but  rise  again  to  9,000  or 
10,000  feet  in  the  extreme  south.  Along  the  west  coast 
the  mountains  are  lower,  their  peaks  rising  from  5,000 
feet  near  the  Orange  to  13,700  feet  in  the  volcanic  Cam- 
eroon Mountains  at  the  head  of  the  Gulf  of  Guinea,  and 


THE   SURFACE    OF   THE    LAND. 


177 


Central 


Eastern 


Highland 


Tonv 


Desert 


y\ 


w. 


THROUGH    THE     SAHARA     DESERT  (  26°  N.  L AT.  ) 
Kilimanjaro 


ON     PARALLEL    OF    10    S.     LATITUDE 
{Heights  exaggerated   100  times) 

Fig.  76.— Two  Sections  across  Africa. 


^ 


thence  sinking  through  the  highland  west  of  the  Niger 
River  to  about  4,500  feet  above  sea-level. 

The  southern  range  of  the  Atlas  Mountains  in  the  north-west  is 
nearly  twice  as  high  as  the  northern,  and  contains  one  peak  of 
nearly  12,000  feet.  The  plateau  between  these  ranges  has  an 
average  elevation  of  about  3,000  feet.  The  highest  peaks  of  the 
central  tongue  of  highlands  are  about  8,000  feet,  and  of  the  north- 
eastern tongue  about  7,000  feet.  The  main  highland  region  is 
divided  by  mountain  ranges  into  several  basin-shaped  plateaus.  The 
highest,  6,000  to  7,000  feet,  extends,  with  a  width  of  200  miles,  from 
Abyssinia  to  the  equator,  a  distance  of  1,200  miles.  South  of  this, 
the  plateaus  maintain  an  average  height  of  about  3,500  feet,  having 
a  central  elevation  of  about  2,500  feet,  and  a  marginal  altitude  of 
about  4,500  feet. 

The  Sahara  lowland  has  elevations  of  about  1,200 
feet  between  the  central  tongue  and  the  north-western 
highland,  and  on  either  side  of  the  Lake  Chad  depression. 
Lake  Chad  is  about  800  feet  above  the  sea,  while  in  the 
north,  limited  areas  between  Tunis  and  the  Nile  are  de- 
pressed to  about  the  level  of  the  sea,  being  in  some 
places  as  much  as   167  feet  below  it. 

Australia  forms  the  extremity  of  a  relatively  small 
branch  from  the  convex  side  of  the  main  continental 
plateau ;  that  is,  Australia  is  connected  with  Asia  by  a 
system  of  narrow,  complex  wrinkles  in  the  earth's  crust, 


178  PHYSICAL    GEOGRAPHY. 

whose  crests  in  some  localities  do  not  rise  quite  to  sea- 
level,  though  throughout  the  greater  part  of  the  distance 
between  the  continents  the  crests  of  the  wrinkles  pro- 
trude above  the  sea  to  form  the  long  and  generally  narrow 
islands  of  the  Malay  Archipelago. 

The  axis  of  this  whole  region  of  elevations  from 
Asia  to  Tasmania  forms  a  sharply  reversed  curve.  In  the 
north  the  curve  is  concave  toward  the  north-east,  while  in 
the  south  it  is  concave  toward  the  south-west.  The  map 
(page  175)  indicates  that  the  law  of  the  main  continental 
plateau  is  equally  true  of  this  branch ;  namely,  the  land  is 
more  continuous  and  the  plateau  is  higher  on  the  convex 
than  on  the  concave  side  of  its  axis;  thus,  in  the  north 
the  Sunda  Islands  form  a  nearly  continuous  rim  of  land 
on  the  convex  west  side.  They  are  separated  from  each 
other  by  narrow  straits  of  relatively  shallow  water,  and  rise 
in  many  peaks  to  an  elevation  of  12,000  feet.  They  con- 
tain more  active  volcanoes  than  any  other  region  of  equal 
extent  in  the  world.  Gilolo  and  the  Philippines,  on  the 
concave  side,  are  much  more  discontinuous,  are  separated 
by  very  deep  water,  and  contain  peaks  of  only  5,000  to 
8,500  feet.  Volcanism,  though  active  on  this  side,  is  less 
so  than  in  the  Sunda  Islands.  The  southern  part  of  the 
axis  just  reverses  the  high  and  low  sides  of  the  plateau. 
From  New  Guinea  to  Tasmania  the  eastern  and  convex 
side  extends  as  an  almost  continuous  region  of  highland, 
with  peaks  of  13,000  feet  in  New  Guinea,  and  of  over 
7,000  feet  in  the  Australian  Alps;  while  on  the  concave 
side,  the  short  west  coast  of  Australia  is  the  only  land,  and 
is  quite  low,  rising  in  but  two  isolated  instances  to  as  much 
as  3,000  feet  above  the  sea.  In  New  Guinea  and  New 
Zealand  there  are  active  volcanoes;  in  Australia  there  are 
none,  but  signs  of  comparatively  recent  activity  occur  in 
the  east,  and  of  very  ancient  action  in  the  west. 


THE  SURFACE  OF  THE  LAND.         1 79 

Transverse  wrinkles  cross  the  northern  part  of  the  axis  to  form 
the  great  islands  of  Borneo  and  Celebes,  while  across  central  Aus- 
tralia a  broad  transverse  wrinkle  or  swell  carries  the  surface  gradu- 
ally up  to  a  general  elevation  of  nearly  2,000  feet,  and  in  places  of 
over  4,000  feet  above  the  sea.  The  south-east  slope  of  Australia 
drops  off  rapidly  to  form  the  sea  bottom  at  a  depth  of  15,000  feet, 
which  then  rises  gradually  to  the  crest  of  the  sharply  curved  New 
Zealand  wrinkle  parallel  with  the  Australian  coast,  and  reaching  in 
the  volcanic  peaks  of  those  islands  an  altitude  of  12,000  feet  above 
the  sea. 


Indian 


Mt.  Bruce 


Mo  Donnell  Mts 


Aus.  Alps 


Pacific 


^w 


Ocean 


MILES  MILES  \  ~^-      mt,  -       1 

ON     TROPIC     OF     CAPRICORN  \        ^  2 

{Heights  exaggerated  100  timea)  E. 

Fig.  77.— Section  Across  Australia. 

Summary. — Thus,  all  the  grand  divisions  contain  mount- 
ain ranges,  which  are  most  numerous  and  highest  on  the 
side  bordered  by  the  nearly  continuous  highland  rim ;  that 
is,  on  the  west  side  of  the  New  World,  but  the  east  and 
south-east  sides  of  the  Old  World.  Therefore,  the  great 
lowlands  of  the  world  border  the  Atlantic  Ocean,  from 
which  they  are  separated  either  by  no  mountains  or  by 
comparatively  isolated  ranges  of  moderate  elevation.  Ad- 
jacent mountain  ranges  in  all  grand  divisions  are  as  a  rule 
roughly  parallel  with  each  other  and  with  the  general  trend 
of  the  nearest  side  of  the  continental  plateau.  Evidences 
of  volcanic  action  are  found  in  nearly  all  mountain  regions. 
Present  volcanic  action  or  evidences  of  relatively  recent 
action  are  most  common  toward  that  side  of  the  grand 
division  which  constitutes  part  of  the  highland  or  convex 
margin  of  the  continental  plateau ;  while  among  the 
isolated  highlands  on  the  concave  side  of  the  continental 
plateau  evidences  of  very  ancient  volcanic  activity  are 
most  common. 


CHAPTER  XIII. 

THE   STRUCTURE   OF   THE   LAND. 

For   a    thousand   years    in    thy  sight  are  but  as  yesterday  when   it  is  past. — 
Psalm  xc:  4. 

Elements. — The  earth  has  been  examined  to  a  depth 
entirely  insignificant  in  comparison  with  its  diameter,  but 
so  far  as  it  has  been  examined,  it  appears  to  be  composed 
mainly  of  twelve  elements,  or  simple  substances.  These 
elements  compose  3%-ths  of  the  earth's  crust,  and  are: 
oxygen,  silicon,  aluminium,  calcium,  magnesium,  potas- 
sium, sodium,  carbon,  hydrogen,  sulphur,  chlorine,  and  iron. 
The  remaining  yj-^th  of  the  earth's  crust  is  composed  of 
about  sixty  other  elements.  Among  these  rare  elements 
are  all  the  useful  metals,  excepting  iron  and  aluminium. 

Minerals. — With  few  exceptions,  the  twelve  abundant 
elements  do  not  occur  in  a  free  state,  but  in  chemical 
combinations  with  each  other  and  with  the  other  elements. 
The  stony  substances  resulting  from  such  combinations  are 
called  minerals.  The  most  abundant  and  common  min- 
erals are  silica  and  its  compounds.  Next  in  abundance 
are  the  carbonates  of  lime  and  magnesia.  The  oxides  of 
iron  are  almost  as  common,   but  not  so  abundant. 

Rocks. — Most  rocks  are  mixtures  of  two  or  more  kinds 
of  minerals,  the  particles  of  each  being  often  visible  to 
the  naked  eye.  Thus,  the  granites  are  essentially  mixt- 
ures of  feldspar,  quartz,  and  mica;  ordinary  "trap"  rocks, 
or  lava,  of  feldspar  and  pyroxene ;  sandstones  consist 
mainly  of  particles  of  silica;  limestones,  of  carbonate  of  lime, 
(180) 


THE    STRUCTURE    OF    THE    LAND.  l8l 

and  shales  and  slates,  of  silicates  of  alumina,  the  principal 
substance  in  clay.  These  grains  are  usually  joined  to- 
gether by  a  cement  of  some  mineral,  which  differs  more 
or  less  from  the  mineral  particles.  Lime,  which  forms 
the  essential  principle  of  most  artificial  mortars  and 
cements,  is  found  in  very  many  rocks  as  the  natural 
cement  binding  together  the  particles,  while  peroxide  of 
iron  and  silica  serve  this  purpose  in  many  other  instances. 
The  various  colors  of  rocks,  clays,  and  earths  are  very 
generally  due  to  minute  quantities  of  iron  distributed 
through  them  in  various  combinations. 

Soil. — All  rocks  disintegrate  —  that  is,  crumble  to 
pieces — more  or  less  rapidly  when  exposed  to  the  weather ; 
the  process  is  therefore  called  weathering.  In  consequence, 
the  surface  of  the  solid  rock  over  most  of  the  earth  is 
covered  with  a  varying  thickness  of  its  own  loosened  and 
detached  mineral  particles.  This  loosened  mass  constitutes 
the  soil.  The  surface  soil  is  constantly  being  removed 
particle  by  particle — chiefly  by  the  wash  of  rains  to  the 
nearest  stream,  but  sometimes  by  winds  as  dust  — while 
the  rock  beneath  constantly  breaks  into  soil.  The  disinte- 
gration and  removal  of  the  rock  together  constitute  the 
process  of  erosion. 

The  chief  agents  in  the  disintegration  of  rocks  by  weathering  are  : 
solution,  change  of  temperature,  the  beating  of  rain,  gravity,  veg- 
etation, and  winds. 

(i)  Solution. — Some  rocks  are  completely  dissolved  by  percolat- 
ing water,  but  the  majority  are  slowly  broken  up  into  particles  by 
the  solution  of  the  cement  which  binds  together  the  more  insoluble 
grains. 

(2)  Change  of  Temperature. — The  hardest  rocks  are  cracked  by 
the  expansions  and  contractions  accompanying  sudden  changes  of 
temperature.  The  crevices  thus  begun  are  opened  by  repeated  ex- 
pansions of  water  freezing  within  them. 

(3)  The  beating  of  rain  overcomes  the  cohesion  of  the  softer 
rocks,  and  assists  solution  and  frost  by  detaching  loosened  particles. 


182 


PHYSICAL    GEOGRAPHY. 


Fig.  78.— Layers  of  Stratified  Rock. 


(4)  Gravity. — When  the  base  of  a  cliff  is  greatly  eroded,  the 
upper  part  breaks  off  and  falls  from  its  own  weight. 

(5)  Plants  often  pry  apart  rocks  by  the  growth  of  their  roots,  but 
their  chief  aid  to  disintegration  is  by  increasing  the  solvent  power 
of  percolating  water. 

Classes  of  Rocks. — The  solid  rocks  beneath  the  soil 
may  be  divided  according  to  their  structure  or  arrange- 
ment into  two  classes:  stratified  and  unstratified. 


THE   STRUCTURE    OF    THE    LAND.  1 83 

Stratified  Rocks  include  sandstones,  limestones,  and 
shales,  and  occur  as  a  general  rule  nearest  the  surface  of 
the  earth.  They  compose  the  surface  rocks  of  about  nine 
tenths  of  the  land.  In  some  places  their  thickness  is 
known  to  be  very  slight;  in  other  places  there  is  good 
reason  to  think  that  they  extend  to  a  depth  of  at  least 
ten  miles.  The  average  thickness  of  stratified  rocks  over 
the  land  is  probably  between  two  and  three  miles. 

Stratified  Rock  is  characterized  (1)  by  its  arrange- 
ment in  sheet-like  layers,  or  strata,  (Fig.  78),  which  vary 
from  the  thinness  of  a  sheet  of  paper  to  a  thickness  of 
many  feet.  (2)  By  a  more  or  less  thorough  assortment  of 
the  different  minerals,  and  their  collection  into  different 
strata ;  thus,  in  each  stratum  some  one  kind  of  mineral  is 
usually  greatly  in  excess  of  all  other  kinds.  (3)  By  con- 
taining imprinted  on  their  surfaces,  or  imbedded  in  their 
mass,  traces  of  animals  or  plants  which  must  have  existed 
before  the  rock  was  formed.  These  traces  of  former  life 
are  called  fossils.  (4)  By  being  largely  composed  of 
mineral  particles  whose  irregular  shapes  indicate  that  they 
are  merely  fragments  from  some  older  rock  mass. 

These  peculiarities  can  only  be  explained  by  sup- 
posing that  these  rocks  resulted  from  the  gradual  com- 
pacting and  hardening  of  beds  of  sand  or  mud,  which  had 
been  deposited  in  water  as  sediment.  Such  beds  of  sedi- 
ment are  now  forming  in  every  body  of  quiet  water,  and 
are  found  of  every  degree  of  hardness  and  compactness, 
from  that  of  the  softest  mud  to  that  of  the  hardest  rock. 
It  is  certain  that  at  least  most  of  the  stratified  rocks  are 
of  such  origin.  They  are  therefore  called  sedimentary, 
aqueous,   or  fragmental  rocks. 

Formation  of  Sedimentary  Rocks. — The  loose  soil  on  the  surface 
of  the  land  affords  directly  or  indirectly  the  greater  part  of  the  min- 
eral particles  which  compose  the  sediment  collecting  on  sea  and 


184  PHYSICAL   GEOGRAPHY. 

lake  bottoms ;  hence,  the  disintegration  of  the  rocks  into  soil  is  the 
first  step  in  the  formation  of  future  rocks.  Particles  of  sand,  clay, 
and  carbonate  of  lime  predominate  in  most  soils,  but  are  all  mixed 
together  in  endless  confusion.  Through  the  force  of  running  water 
and  of  gravity  the  particles  are  assorted  and  eventually  transported 
to  some  lake  or  to  the  sea,  where  they  are  deposited  in  more  or  less 
distinct  beds.  The  material  of  pure  sand  (silica  or  quartz),  owing  to 
its  hard  and  durable  nature,  disintegrates  very  slowly,  and  thus, 
speaking  generally,  forms  the  largest  and  heaviest  fragments  in 
sediment.  The  heaviest  panicles,  of  course,  sink  soonest;  hence, 
sand  predominates  in  the  deposit  nearest  the  shore,  which  gradually 
consolidates  into  sandstones  of  different  varieties.  The  material  of 
clay  is  derived  from  the  chemical  decomposition  of  feldspar,  arid  is 
in  very  fine  particles ;  hence,  it  does  not  sink  so  soon  as  the  heavier 
particles  of  silica,  but  is  carried  to  greater  distances  from  the  shore, 
where  it  predominates  in  the  sediments  and  consolidates  into  various 
kinds  of  shale.  Fragments  of  carbonate  of  lime  are  also  carried 
down  from  the  land,  and  are  deposited  according  to  their  size  and 
weight ;  but  as  this  mineral  is  more  or  less  soluble,  these  fragments 
grow  smaller  the  longer  they  are  in  the  water.  Much  carbonate  of 
lime,  therefore,  reaches  the  sea  in  solution,  and  is  generally  distrib- 
uted by  ocean  currents  as  a  chemical  ingredient  of  the  water.  From 
this  ingredient  aquatic  plants  and  animals  derive  material  for  their 
shells  and  skeletons.  Upon  the  death  of  the  organisms,  these  sink 
toward  the  bottom  as  sediment.  Life  is  so  abundant  in  many  parts 
of  the  sea  that  where  the  water  is  shallow  these  shell  fragments  ac- 
cumulate on  the  bottom  faster  than  the  water  can  dissolve  them. 
When  this  occurs  in  regions  where  but  little  sand  or  clay  sediments 
are  accumulating,  beds  of  mud  of  nearly  pure  carbonate  of  lime 
may  be  formed,  similar  to,  but  not  exactly  like,  the  organic  deep  sea 
oozes.  The  consolidation  of  such  beds  produced  by  far  the  greater 
part  of  our  limestones. 

Unstratified  Rocks  underlie  the  stratified  rocks  and 
extend  indefinitely  into  the  interior  of  the  earth.  In  some 
places  the  unstratified  break  through  the  stratified  rocks, 
thus  forming  the  surface  rocks  over  about  one  tenth  of  the 
land.  Unstratified  rocks  include  the  granites;  the  finer 
grained  rocks,  called  trap  rocks,  as  trachyte,  basalt,  and 
obsidian ;  and  the  still  finer  grained  modern  lavas. 


THE  STRUCTURE  OF  THE  LAND.        1 85 

Peculiarities. — The  texture  of  unstratified  rocks  is 
peculiar  in  being  either  smooth  and  glassy,  or,  if  granular, 
the  grains  or  particles  have  more  or  less  distinctly  the  reg- 
ular shape  and  structure  of  the  crystals  peculiar  to  the 
mineral  of  which  they  are  composed.  Now,  melted  rock 
in  cooling  assumes  this  same  glassy  or  crystalline  texture, 
and  this,  with  the  absence  of  fossils,  suggests  that  heat 
was  an  essential  agent  in  the  formation  of  the  unstratified 
rocks.      Hence,  they  are  often  called  igneous  (fire)  rocks. 

An  igneous  origin  is  also  indicated  by  the  manner  in 
which  unstratified  rocks  occur;  namely,  (1)  as  structure- 
less and  irregular  shaped  masses,  forming  the  core  of 
some  mountain  chains;  (2)  as  lava  "  dikes,"  filling  great 
fissures  across  the  beds  of  the  stratified  rocks,  as  though 
lava  had  been  forced  into  these  fissures  from  below  when 
in  a  melted  state ;  and  (3)  as  lavas,  overlying  stratified 
rocks,  as  though  they  had  welled  up  through  a  volcanic 
vent  or  a  fissure,  and  spread  out  over  the  surrounding 
surface  before  cooling. 

Not  infrequently  dikes  and  sheets  of  unstratified  rock  are  wholly 
composed  of  regular  columns  arranged  side  by  side  and  closely  fit- 
ting together.  Now,  the  contraction  of  a  bed  of  fine  mud,  as  it 
dries  in  the  sun,  frequently  causes  cracks  to  traverse  its  surface  in  all 
directions,  subdividing  that  surface  into  more  or  less  regular  shaped 
areas,  which,  as  the  cracks  often  penetrate  deeply  into  the  mud  bed, 
are  really  but  the  ends  of  a  series  of  columns  similar  to  those  found 
in  the  rock.  Hence,  the  columnar  structure  of  rocks  is  thought  to 
have  resulted  from  a  similar  cause,  namely,  the  contraction  of  the 
rock  mass  in  cooling. 

Metamorphic  Rocks. — Certain  rocks,  including  slate, 
quartzite,  marble,  etc.,  possess  more  or  less  distinctly 
the  stratified  arrangement  of  the  sedimentary  rocks, 
with  the  crystalline  texture  of  the  igneous  rocks.  They 
are  often  of  aqueous  origin,  but  their  original  frag- 
mental  texture  has  subseqv^ntiv  been  changed  more  or  less 

P.  G— 11. 


1 86  PHYSICAL    GEOGRAPHY. 

perfectly  to  a  crystalline  texture ;  they  are  therefore  called 
changed,  or  metamorphic,  rocks. 

The  causes  of  this  change  or  metamorphism  are 
heat,  moisture,  and  pressure,  while  the  change  is  greatly 
facilitated  by  the  presence  of  certain  common  minerals. 
In  order  that  the  mineral  molecules  in  a  fragmental  rock 
may  assume  a  crystalline  arrangement,  a  certain  freedom 
of  movement,  as  exists  in  pasty  substances,  is  necessary. 
Under  ordinary  conditions  it  requires  a  temperature  of 
nearly  3,ooo°  to  melt  or  lique-fy  rocks,  but  when  thus 
melted,  all  stratification  would  disappear.  When,  how- 
ever, rocks  under  great  pressure  are  heated  in  the  pres- 
ence of  even  a  very  minute  quantity  of  water  so  placed 
that  it  can  not  expand  into  steam,  and  especially  if  certain 
minerals  are  in  solution  in  the  water,  the  rock  begins  at 
temperatures  of  only  between  200°  and  3000  to  pass  into  a 
state  which  seems  to  allow  of  crystallization  and  of  new 
chemical  combinations,  but  does  not  destroy  the  stratifi- 
cation. 

As  the  earth  is  penetrated,  it  becomes  warmer  at  a  rate  which  at 
a  depth  of  y/%  miles  would  produce  a  temperature  3000  above  that 
at  the  surface.  Now,  in  some  places  the  stratified  rocks  are  more 
than  y/t  miles  thick ;  it  is  therefore  evident  that  the  temperature  of 
the  bottom  strata  in  such  places  must  be  at  least  3000  higher  than 
when  these  bottom  strata  formed  the  surface  of  the  deposit.  As  the 
weight  of  the  sediment  above  would  exert  great  pressure,  and  as  all 
rocks  contain  more  or  less  water  percolating  through  them,  which 
water,  too,  is  frequently  impregnated  with  just  the  necessary  minerals, 
it  is  more  than  probable  that  even  at  this  depth  the  conditions  are 
generally  favorable  for  the  partial  crystallization  or  metamorphism 
of  the  lower  strata.  As  the  thickness  of  the  deposit  increased  by 
the  continued  accumulation  of  sediment  on  top,  the  heat,  pressure, 
and  metamorphism  in  the  lower  strata  would  increase,  while  the 
strata  above  would  begin  to  crystallize,  until  finally  the  heat  in  the 
lower  strata  might  become  so  great  as  to  destroy  all  trace  of  stratifi- 
cation, and  convert  the  rocks  into  true  unstratified  or  igneous  rocks. 
Since  metamorphism  takes  place  only  at  great  depths,  and  since 


THE    STRUCTURE    OF    THE    LAND.  1 87 

metamorphic  rocks  could  not  have  retained  the  stratified  structure 
had  they  ever  been  rendered  soft  enough  to  admit  of  their  being 
forced  through  the  overlying  rocks  to  the  earth's  surface,  it  follows 
that  crystalline  rock,  having  a  stratified  structure,  occurs  at  the 
earth's  surface  only  when  it  has  been  denuded,  or  laid  bare,  by  the 
gradual  disintegration  and  removal  of  a  great  thickness  of  rock 
which  once  covered  it.  The  occurrence  of  metamorphic  rock  at  the 
surface  of  the  earth  is  thus  of  itself  an  indication  of  extensive  erosion, 
or  denudation   (see  Chapter  XVIII). 

The  Primitive  Rock,  or  that  which  first  formed  over 
the  earth's  surface  by  the  gradual  cooling  of  the  molten 
globe,  must  have  resembled  the  present  igneous  rock  in 
containing  no  fossils,  in  being  unstratified,  and  in  having 
a  crystalline  or  glassy  texture.  But  nearly  all  crystals 
contain  numberless  microscopic  cavities.  In  slags  and 
lavas,  which  are  known  to  have  solidified  slowly,  like  the 
primitive  rock,  from  a  melted  state,  these  cavities  are 
filled  with  the  mineral  of  the  crystals,  but  in  a  glassy  con- 
dition. In  crystals,  however,  which  have  been  produced 
artificially  by  a  process  similar  to  that  which  resulted  in 
metamorphism,  many  of  the  cavities  contain  nothing  but 
water.  Now,  in  most  of  the  unstratified  or  igneous 
rocks,  many  of  these  crystal-cavities  contain  water,  which 
indicates  that  they  are  aqueous  rocks  which  have  under- 
gone complete  metamorphism.  In  fact,  it  is  probable 
that  none  of  the  primitive  rock  now  remains  on  the  earth's 
surface  in  its  original  position  and  condition,  but  that  dur- 
ing the  ages  which  have  elapsed  since  its  formation  this 
rock  afforded  the  mineral  particles  to  soil  and  sediment, 
which  eventually  completely  covered  its  surface,  and 
through  progressive  changes  became  successively  the  strat- 
ified-fragmental,  the  metamorphic,  and  the  unstratified- 
crystalline  rocks  which  compose  the  present  surface,  and 
which  are  now  supplying  through  similar  processes  these 
same  particles  to  other  similar  cycles  of  change. 


1 88  PHYSICAL    GEOGRAPHY. 

The  Land  was  once  Submerged. — Since  most,  if  not 
all,  of  the  rocks  of  the  land  are  thus  but  more  or  less  com- 
pletely changed  sediments,  it  follows  that  the  present  land 
must  at  some  time  have  been  entirely  under  water  in 
order  that  the  sediment  might  accumulate.  All  of  the 
land  could  not  have  been  under  water  at  one  time,  how- 
ever, for  it  has  been  seen  that  the  bulk  of  the  sediment 
comes  from  the  disintegration  of  an  adjacent  land  surface. 
Hence,  we  conclude  that  adjacent  areas  of  the  land  have 
alternately  been  depressed  below  and  then  elevated  above 
the  surface  of  the  sea  —  perhaps  many  times — the  area 
above  the  sea  supplying  the  material  which  was  deposited 
on  the  adjacent  depressed  area,  which,  when  subsequently 
elevated,  supplied  material  for  the  sediment  collecting  in 
surrounding  depressed  regions. 

This  makes  it  evident  that  no  stratum  of  sedimentary  rock  is 
continuous  over  very  wide  regions,  but  that  each,  considered  as  a 
whole,  is  a  great  cake,  whose  thickness  decreases  gradually  in  all 
directions  from  the  region  which,  at  the  time  of  its  deposit,  was 
nearest  to  the  source  of  supply,  and  hence  received  the  most  sedi- 
ment. 

Disturbed  and  Faulted  Strata. — That  such  movements 
of  elevation  and  depression  are  possible,  is  proved  by  the 
fact  that  in  many  localities  the  coast  regions  of  the  land 
are  observed  to  be  very  gradually  rising  above  or  sinking 
beneath  the  adjacent  water  surface,  while  such  movements 
are  proved  to  have  taken  place  in  regions  far  from  the 
present  coasts  by  the  position  of  the  strata.  Sediment 
will  not  rest  at  all  on  a  steeply  sloping  bottom,  and  its 
deposition  tends  to  lessen  gentle  slopes.  It  is  therefore 
certain  that  most  of  the  sedimentary  strata  were  originally 
nearly  or  quite  horizontal.  As  actually  found,  however, 
the  strata  are  seldom  exactly  level ;  they  every-where  show 
more  or  less  distinctly  traces   of  tilting   or   curvature,  as 


THE    STRUCTURE    OF   THE    LAND. 


189 


though  they  had  been  thrown  from  their  original  hori- 
zontal position  into  a  series  of  great,  wave-like  undulations. 
The  surface  at  the  crest  and  trough  of  such  a  rock  wave 
was  of  course  elevated  and  depressed  respectively  when 
the  movement  took  place.  In  some  places  the  waves  are 
short  and  high,  in  which  case  the  strata  composing  their 
sides  slope,  or  dip,  at  a  steep  angle ;  in  other  cases,  the 
waves  are  so  long  and  flat  that  the  dip  of  the  strata  is  im- 
perceptible.     Frequently  the  strata  are  found  to  be  broken 


Fig.  79. 

across,  and  the  strata  on  one  side  of  the  break  to  have 
been  upheaved  above  or  depressed  below  the  correspond- 
ing strata  on  the  other  side.     This  is  called  a  fault. 

It  seldom  happens  that  the  strata  can  be  actually  seen  continu- 
ously from  the  crest  to  the  trough  of  a  wave ;  they  generally  dip 
down  out  of  sight  into  the  earth  in  one  direction,  while  the  top  of  the 
wave  has  been  carried  away  piecemeal  by  erosion,  leaving  only  the 
ragged  edges  of  the  strata  to  compose  the  earth's  surface.  The 
shape  of  the  wave,  however  (shown  by  the  dotted  lines  in  the  dia- 
gram, Fig.  79),  is  indicated  by  the  various  dips  of  the  adjacent  strata. 
Erosion  has  in  the  same  manner  quite  generally  carried  away  the 
upheaved  side  of  faults,  so  that  their  position  is  indicated  by  a  sud- 
den change  in  the  character  of  the  rock  rather  than  by  a  sudden 


I90  PHYSICAL    GEOGRAPHY. 

change  in  elevation  in  the  earth's  surface.  Many  circumstances, 
such  as  the  enormous  erosion,  prove  that  these  faults  and  flexures 
of  the  strata  were  not  produced  by  great  single  movements,  but  that 
each  is  the  aggregate  result  of  thousands  of  slight  movements  of  a 
few  inches  or  a  tew  feet,  occurring  at  irregular  but  very  long  intervals 
of  time.  These  slight  movements  are  still  taking  place  in  all  parts 
of  the  world,  and  are,  as  we  shall  see  later,  the  cause  of  earth- 
quakes. There  is  probably  no  locality  in  which  these  movements 
are  always  in  the  same  direction,  either  upward  or  downward ;  but, 
in  general,  the  convex  or  Pacific  side  of  the  continental  plateau 
seems  to  be  slowly  rising,  while,  with  certain  local  exceptions,  the 
concave  or  Atlantic  side  seems  to  be  gradually  sinking. 


Fig.  80 — Unconformable  Strata. 

Unconformity  of  the  Strata.— -In  some  places  a  series 
of  strata  A  (Fig.  80),  having  a  certain  dip,  rest  directly 
upon  the  eroded  surface  of  another  series,  having  either 
the  same  dip  (B),  or  an  entirely  different  one  (C).  The 
two  series  are  then  said  to  be  unconformable. 

Such  a  position  indicates  several  movements  of  the  earth's  crust; 
thus,  (1)  an  upward  movement  of  the  sediments  B  or  C  to  bring 
them  above  the  water  that  they  might  be  exposed  to  the  weather  and 
eroded ;  (2)  a  downward  movement  to  allow  the  deposit  of  the  sedi- 
mentary rock  A  beneath  the  water,  and  (3)  an  upward  movement  to 
convert  this  deposit  into  the  present  land. 

Relative  Age  of  Rocks. — When  the  relative  position 
of  rocks  has  not  been  greatly  disturbed  by  subsequent 
movement,  it  indicates  the  order  of  formation  or  relative 
age  of  the  rocks,  the  highest  being  the  youngest  and  the 
lowest  the  oldest.  But  this  method  involves  the  direct 
comparison  of  the  position  of  rocks,  and  therefore  applies 


THE    STRUCTURE    OF    THE    LAND. 


only  to  rocks  in  the  immediate  neighborhood  of  each 
other.  To  determine  the  relative  age  of  the  rocks  com- 
posing widely  separated  regions — on  different  continents, 
for  instance,  or  on  opposite  sides  of  the  same  continent — 
some  other  method  has  to  be  used,  since  it  is  never  possi- 
ble to  trace  the  strata  from  one  region  directly  to  the 
other  over  the  intervening  distance.  The  only  known 
method  for  identifying  the  relative  periods  in  geological 
time,  at  which  rocks  in  widely  separated  regions  were 
formed,  is  by  a  comparison  of  their  fossils. 


Fig.  81.— Rock  Fragments,  showing  Embedded  Fossils. 

Fossils. — The  harder  parts  of  land  or  aquatic  animals 
and  plants  are  sometimes  buried  in  adjacent  accumulations 
of  sediment  and  preserved  for  long  ages.  When  at  last 
they  decay,  a  hollow  mold  having  their  shape  or  outline 
is  left  in  the  hardened  deposit,  and  is  gradually  filled  up 
solid  by  the  precipitation  of  some  mineral  in  solution  in 
the  water  percolating  through  the  deposit.  Thus  a  frag- 
mentary record  of  the  forms  of  life  which  existed  at  the 
time  each  layer  of  sediment  was  deposited  is  preserved 
within  the  rock  stratum,  either  as  the  organic  remains 
itself,  its  empty  mold,  or  as  a  stone  cast  (Fig.  81)  filling 
up  this  mold,  until  metamorphism  effaces,  more  or  less 
completely,  both  the  lines  of  stratification  and  the  fossil 
contents  of  the  rocks. 


I92  PHYSICAL    GEOGRAPHY. 

Sometimes  the  precipitation  from  the  percolating  water  replaces 
the  organism  particle  for  particle  as  it  decays,  thus  preserving  in 
stone  all  the  delicate  internal  structure  of  the  organism.  Such 
fossils  are  called  petrifactions.  In  other  cases,  only  a  portion  of  the 
substances  liberated  by  the  decay  of  the  organism  escapes,  and  the 
residue  recombines  into  a  new  substance  which  may  or  may  not 
retain  the  outline  of  the  organism.  Coal,  asphalt,  petroleum,  and 
"natural  gas  "  are  the  new  substances  which,  under  different  circum- 
stances, result  from  this  process  (page  369). 

Identification  of  Relative  Age  of  Strata  by  Fos- 
sils.— Careful  examination  of  the  fossils  in  thick  series  of 
stratified  rocks,  whose  relative  age  is  indicated  by  the  rel- 
ative position  of  the  strata,  reveals  that  the  fossils  at  the 
bottom  are  not  quite  the  same  as  those  at  the  top  of  the 
series.  As  the  series  is  ascended,  different  kinds  of  fossils 
gradually  disappear,  while  other  kinds  gradually  make 
their  appearance,  and  are  in  turn  replaced  by  newer  forms. 
It  thus  appears  that  each  stratum  contains  a  few  kinds  of 
fossils  not  found  in  any  other  strata.  These  peculiar 
fossils,  though  not  always  the  most  numerous,  are  called 
the  type  fossils  of  that  stratum.  When  similar  type  fossils 
are  found  in  widely  separated  regions,  they  always  succeed 
each  other  in  the  same  general  order;  that  is,  the  older 
fossils  in  one  region  are  also  the  older  in  other  regions.  It 
has  thus  been  established  that  the  gradual  changes  in  the 
forms  of  life  in  the  past  have  taken  place  in  the  same 
general  order  all  over  the  world,  and  that  similarity  of 
type  fossils  serves  to  identify  corresponding  strata  in 
widely  separated  regions,  and  to  afford  a  clue  to  the  relative 
ages  of  rocks. 

It  is  probable  that  these  changes  in  life  forms  took  place  more 
rapidly  in  some  regions  than  in  others ;  hence,  the  occurrence  of 
similar  type  fossils  in  widely  separated  regions  does  not  necessarily 
indicate  that  these  organisms  lived  at  exactly  the  same  time,  but  that 
they  lived  when  the  changes  of  life  forms  had  reached  correspond- 
ing periods  in  the  two  regions. 


THE  STRUCTURE  OF  THE  LAND. 


193 


Classification  of  Rocks. — The  rocks  which  expose 
their  edges  at  some  point  or  other  of  the  earth's  surface 
are  classified  according  to  their  relative  age.  The  thou- 
sands of  strata  are  divided  into  five  great  groups,  each  of 
which  marks  an  era  of  time:  (1)  Azoic  (lifeless)  or 
Eozoic  (dawn  of  life),  the  oldest,  in  which  all  the  strata 
yet  found  are  so  completely  metamorphosed  that  the  fossils 
are  either  effaced  entirely  or  rendered  unrecognizable;  (2) 
Paleozoic  (ancient  life),  or  Primary,  in  which  most  of  the 
strata  have  been  metamorphosed,  but  some  retain  their 
fossils  as  the  most  ancient  recognizable  forms  of  life ;  (3) 
Mesozoic  (middle  life),  or  Secondary,  in  which  metamor- 
phosed strata  are  frequent;  (4)  Cainozoic  (recent  life),  or 
Tertiary,  in  which  metamorphism  is  quite  exceptional ; 
and  (5)  Post  Tertiary,  or  Quaternary,  which  includes  fossils 
of  the  present  forms  of  life,  and  in  which  no  metamor- 
phosed strata  are  found.  The  strata  composing  these 
groups  are  subdivided  into 
systems,  each  marking  a 
period  of  time  ;  these  into 
series,  marking  epochs  of 
time  ;  and  these  again  into 
stages,  marking  ages  of 
time ;  while  the  stages 
are  composed  of  beds  or 
individual  strata. 

Geological  Time. — If 
any  of  the  numerous 
changes  in  the  past  which 
are  indicated  by  the  study 
of  the  rocks  be  compared 
with  the  rate  at  which 
similar  changes  are  taking 
place  in  the  present,   the 


Eras  of  Time. 


Periods  of  Time. 


Quaternary. 

Tertiary 

or 
Cainozoic. 
Secondary 

or 
Mesozoic. 


Primary 

or 
Paleozoic. 


Azoic 

or 

Eozoic. 


Recent 
Pleistocene. 
Pliocene. 
Miocene. 
Eocene. 
Cretaceous. 
Jurassic. 
Triassic. 
Permian. 
Carboniferous. 
Devonian. 
Silurian. 
Cambrian. 
L  Archaean. 

Not  subdivided,  because 
as  all  stratification  and 
fossils  have  been  de- 
stroyed by  metamorpho- 
sis, nothing  remains  to 
determine  the  relative 
ages  of  different  parts  of 
*>  the  group. 


194  PHYSICAL    GEOGRAPHY. 

conclusion  is  irresistible  that  geological  time  must  be  very, 
very  long.  If  only  the  small  thickness  of  sediment  de- 
posited in  one  year  by  even  the  muddiest  water  be 
compared  with  the  very  great  average  thickness  of  the 
sedimentary  rocks,  one  becomes  convinced  that  many 
thousands  or  even  millions  of  years  have  been  required 
for  these  rocks  to  accumulate.  There  is  no  way  to  deter- 
mine the  exact  length  of  geological  time.  Some  circum- 
stances indicate  that  at  least  100,000,000  years  must 
have  elapsed  since  the  oldest  known  sedimentary  rocks 
were  deposited  ;  other  circumstances  indicate  that  it  could 
not  have  been  more  than  3,000,000  years,  but  neither  of 
these  estimates  is  accurate — the  time  may  be  greater,  or  it 
may  be  less.  All  that  can  safely  be  affirmed  is  that  the 
fragmentary  record  of  the  earth's  history  which  the  sedi- 
mentary rocks  afford,   covers  a  very  long  period  of  time. 

It  is  perhaps  impossible  for  the  human  intellect  to  grasp  the  lapse 
of  time  comprehended  in  the  expression  "one  million  years."  By 
a  great  effort  of  memory,  an  old  man  may  appreciate  the  length  of 
not  much  more  than  half  a  century ;  and  yet  if  half  a  century  be 
represented  by  a  distance  of  three  inches,  a  million  .years  would  be 
represented  by  one  mile. 


CHAPTER  XIV. 

THE    WATER    OF   THE    LAND SPRINGS. 

He  sendeth  the  springs  into  the  valleys,  which  run  among  the  hills.  They  give 
drink  to  every  beast  of  the  field. — Psalm  civ:  io,  ii. 

The  vapor  of  the  atmosphere,  through  its  condensa- 
tion into  rain,  snow,  dew,  etc.,  supplies  all  the  water 
encountered  on  the  surface  of  the  land.  This  may  be 
classified  according  to  its  manner  of  occurrence,  as  springs, 
streams,  glaciers,  and  lakes. 

Permeability  of  Rocks. — All  rocks  can  absorb  more 
or  less  water.  Clay  and  fine  grained,  compact  rocks, 
though  they  may  contain  water,  do  not  allow  it  to  escape 
readily,  and  are  therefore  said  to  be  impermeable.  A 
layer  of  soil,  sand,  or  coarse  grained,  loosely  cohering, 
or  much  fissured  rock,  on  the  contrary,  allows  water  to 
pass  through  it  copiously,  or  is  said  to  be  permeable. 

Invisible  cavities  between  the  mineral  particles,  and  visible  fissures 
make  up  from  one  sixtieth  to  one  half  the  bulk  of  most  rocks. 
Water  is  absorbed  or  forced  into  these  cavities  by  its  own  weight 
(gravity)  and  by  capillary  attraction  (adhesion).  When  the  cavities 
are  very  minute,  capillary  attraction  is  stronger  than  gravity,  and 
holds  the  water  fast  in  the  cavities,  making  the  rock  impermeable 
though  it  contains  water.  When  the  cavities  are  large,  gravity  is 
stronger  than  capillary  attraction,  and  the  water  sinks  through  the 
cavities  and  escapes  from  the  rock  below. 

Surface  Springs  and  Wells. — When  the  surface  rocks 
are  permeable,  a  large  part  of  the  rain  or  snow  water 
sinks  through  them  until  it  reaches  and  saturates  an  im- 
permeable stratum.     Being  unable  to  escape  through  this 

(195) 


196 


PHYSICAL    GEOGRAPHY. 


RELATIVELY     HIGH      LAND 


•V^^^civ;::;;^;:;-  / mpermeable  ;.        Stratum 
Fig.  82. 

stratum,  the  water  accumulates  in  and  saturates  the  over- 
lying rocks  to  a  height  (s,  Fig.  82)  where  its  own  pres- 
sure forces  it  to  move  slowly  along  the  depressions  in  the 
surface  of  the  impermeable  stratum.  If  this  stratum  is  at 
such  a  slight  depth  that  its  edges  crop  out  on  the  sides  or 
in  the  bottom  of  the  adjacent  valleys,  the  water  issues 
along  this  line  of  outcrop  as  surface  springs.  During  wet 
weather  the  water  collects  in  the  rocks  above  the  imper- 
meable stratum  faster  than  it  escapes  at  the  springs;  the 
upper  limit  of  saturation  (s)  therefore  rises,  its  elevation 
being  approximately  marked  by  the  surface  of  the  water 
in  wells.  During  dry  weather  the  continued  flow  of  the 
springs  causes  the  limit  of  saturation  to  fall.  If  it  should 
fall  below  the  line  s  all  the  springs  would  dry  up,  although 
a  well  (wi)  penetrating  below  this  line  would  still  supply 
water,  while  a  shallower  well  (w2)  would  be  dry.  When 
the  limit  of  saturation  is  very  high  (as  si),  the  increased 
pressure  frequently  forces  the  water  to  the  surface  at  unus- 
ually high  levels,  forming  wet  weather  or  intermittent 
springs,  which  flow  only  until  the  excessive  pressure  is  re- 
lieved by  the  lowering  of  the  limit  of  saturation. 

Deep-seated  Springs  and  Artesian  Wells. — When 
inclined  strata  outcrop  at  the  earth's  surface,  and  are 
arranged  in  such  a  manner  that  permeable  strata  are  in- 
closed between  impermeable  strata,  the  rain  or  snow  water 
which  sinks  into  the  permeable  strata  at  their  outcrop  is 
confined  in  these  strata  by  the  impermeable  beds  above 
and  below.     To  whatever  depths  the  permeable  beds  may 


SPRINGS. 


197 


descend,  this  water  necessarily  follows,  and  may  in  this 
way  travel  underground  for  many  miles  and  reach  depths 
of  thousands  of  feet,  until  stopped  by  the  gradual  thin- 
ning out  of  the  stratum,  by  its  sudden  ending  in  a  close 
fault,  or  by  the  high  temperature  of  the  earth  at  extreme 
depths.  When  the  descent  of  the  water  is  stopped  from 
any  cause,  the  strata  gradually  become  saturated  up  to 
the  lowest  level  of  their  outcrop.  The  water  in  the  sat- 
urated strata  toward  the  lower  end  of  the  incline  is  of 
course  pressed  upon  by  the  weight  of  the  water  toward 
the  upper  end.     This  pressure  is  often  great  enough  to 


Level    of    Outcrop 


Outcrop 


Fig.  83. 


force  the  water  to  clear  a  passage  for  itself  through  some 
small  fissure  or  other  channel  in  the  overlying  imperme- 
able beds,  and,  rising  in  this  channel,  to  gush  forth  as  a 
deep  seated  spring  or  natural  fountain  in  a  region  where 
the  surface  is  at  a  lower  level  than  the  surface  region  where 
the  permeable  beds  receive  their  supply  of  water.  An  arti- 
ficial channel  of  this  kind,  produced  by  drilling  a  hole 
through  the  impermeable  strata,  constitutes  an  artesian 
well,  so  called  from  its  early  use  in  Artois,   France. 

Theoretically,  the  water  will  rise  through  the  well  to  the  same 
height  as  the  outcrop  of  the  permeable  strata ;  but  the  obstruction 
offered  to  the  flow  of  water  by  the  permeable  rock  and  the  leakage 
through  the  confining  strata  considerably  reduces  the  height  to 
which  the  water  will  rise.  Where  these  two  factors  are  very  small, 
the  water  has  been  found  to  rise  to  the  surface  when  the  surface  at 
the  well  is  about  as  many  feet  below  the  surface  at  the  outcrop,  as  the 


I98  PHYSICAL    GEOGRAPHY. 

two  localities  are  distant  from  each  other  in  miles.  If  this  difference 
in  height  is  much  greater,  the  water  may  rise  from  the  well  into  the 
air  as  a  jet  or  fountain.  Artesian  wells  are  now  very  common — 
almost  every  city  in  America  and  Europe  containing  one  or  more. 
They  are  especially  valuable  in  regions  having  a  dry  climate,  as  in 
the  western  portion  of  the  Union  and  the  desert  portion  of  Algeria. 
These  wells  vary  greatly  in  depth  in  different  localities:  one  in 
Berlin  is  4,172  feet  deep;  one  in  St.  Louis,  Mo.,  is  3,843  feet;  in 
Budapest,  Hungary,  over  3,000  feet ;  in  Cincinnati,  O.,  2,408  feet ; 
in  Louisville,  Ky.,  2,086  feet. 

Use  of  Springs. — By  absorbing  and  temporarily  re- 
taining a  large  part  of  the  rainfall,  the  permeable  rocks 
prevent  devastating  floods  which  otherwise  would  accom- 
pany every  heavy  rain,  while  by  the  gradual  surrender 
of  the  absorbed  water  in  springs  the  supply  of  fresh  water 
at  the  earth's  surface  is  maintained  through  ordinary 
seasons  of  drought. 

Devastating  floods  sometimes  occur,  it  is  true,  but  are  almost  in- 
variably due  to  the  rapid  melting  of  snow  by  warm  rains  at  a  time 
when  the  underlying  soil  is  either  completely  saturated  or  is  rendered 
temporarily  impermeable  by  frost.  In  ordinary  summer  droughts, 
such  streams  as  the  Ohio  and  upper  Mississippi,  which  are  not  sup- 
plied at  that  season  by  melting  snows,  contain  only  spring  water. 
Should  the  drought  continue  long  enough,  the  springs  would  exhaust 
the  underground  supply,  and  such  streams  would  dry  up. 

The  temperature  of  the  water  in  springs  is  nearly  con- 
stant throughout  the  year.  It  frequently  seems  warm  in 
winter  and  cool  in  summer,  but  it  is  really  the  temperature 
of  the  air  and  surface  rock  which  varies — the  spring  water 
seeming  warm  or  cool  in  comparison.  The  temperature  of 
different  springs,  however,  varies  greatly.  It  is  rarely  less 
than  400  Fahrenheit,  but  may  range  upward  to  the  boiling 
point.  When  the  temperature  of  the  water  is  much 
higher  than  the  mean  temperature  of  the  surface  rocks  in 
the  vicinity,  the  spring  is  called  a  warm  or  thermal  spring. 
Springs  slightly  warmer  than  the  surface  rocks  are  com- 
mon, and   springs   much  warmer  are  by  no  means  rare. 


%-..,. 


Artesian  Well  at  Prairie  du  Chien,  Wis. 


(199) 


200 


PHYSICAL    GEOGRAPHY. 


In  the  region  of  volcanic  rocks  between  the  Rocky  Mountains 
and  Sierra  Nevada,  perceptibly  warm  springs  are  the  rule.  East  of 
the  Rocky  Mountains  they  are  more  exceptional,  but  are  found  in 
nearly  every  state ; — a  very  few  are  named  below : 


Temp. 

Mean 

</) 

Flow 

Name. 

Locality. 

of 

Temp. 

u 

Gal.  Per 

Spring 

Surf. 

W 

Hour. 

Lebanon   Springs, 

Columbia  Co.,  N.  Y. 

75° 

46° 

29° 

30,000 

Warm 

Bath  Co.,  Va. 

98 

46 

52 

360,000 

Sweet              " 

Monroe  Co.,  W.  Va. 

79 

46 

33 

48,000 

Warm              " 

Meriwether  Co.,  Ga. 

90 

60 

30 

84,000 

Hot 

Garland  Co.,  Ark. 

157 

62 

95 

20,I00 

Palmyra          " 

Jefferson  Co.,  Wis. 

72 

46 

26 

Blankenships  " 

Texas  Co.,  Mo. 

75 

57 

18 

2,000 

Spring  water  derives  its  temperature  from  the  rocks  through  which 
it  percolates,  and  the  rocks  at  a  very  slight  depth  cease  to  be 
affected  by  the  daily  and  seasonal  variations  of  surface  temperature. 
Since  the  rocks  in  non-volcanic  regions  become  warmer  at  an 
average  rate  of  i°  for  each  50  feet  of  increased  depth,  and  since  the 
water  percolates  very  slowly,  it  has  time  to  acquire  the  rock  tempera- 
ture during  its  downward  passage.  It  follows  more  or  less  open 
channels  in  its  journey  upward  or  outward  to  springs,  and  flowing 
more  rapidly  does  not  lose  all  of  its  acquired  heat.  In  non-volcanic 
districts  the  excess  of  temperature  of  spring  water  affords  a  very 
rough  approximation  of  the  depth  from  which  it  has  come.  The 
Arkansas  Hot  Springs  have  an  excess  of  950,  and  must  come  from 
a  depth  of  nearly  a  mile.  The  water  of  deep  artesian  wells  is 
almost  always  perceptibly  warm :  that  at  St.  Louis  has  a  tempera- 
ture of  1050,  and  that  at  Louisville  of  76^°,  the  mean  temperature 
of  the  air  at  those  places  being  55°  and  570  respectively. 

Spring  Water. — In  percolating  through  the  rocks,  the 
water  is  constantly  dissolving  and  carrying  along  with  it 
soluble  minerals.  In  addition  to  this,  it  is  constantly  caus- 
ing chemical  changes,  by  which  new  and  soluble  substances 
may  be  made  from  insoluble  minerals.  Frequently  these 
new  substances  are  gases,  with  which  the  water  is  charged 
when  it  arrives  at  the  surface.     The  most  common  gas 


SPRINGS.  20 1 

thus  produced  is  carbonic  acid,  which,  escaping  in  minute 
bubbles,  causes  the  usual  "  sparkle  "  of  spring  water.  The 
gas  sulphuretted  hydrogen  causes  the  disagreeable  odor 
of  most  "  sulphur"  springs.  Thus,  strictly  speaking,  all 
springs  are  mineral  springs,  but  only  those  are  usually  so 
called  in  which  the  mineral  or  gaseous  contents  impart  a 
perceptible  taste  or  peculiar  medicinal  quality  to  the  water. 

The  minerals  which  most  commonly  occur  in  spring  water  are : 

Carbonate  of  lime  )  .  .  . 

y  making  temporary  hard  water. 

"  "  magnesia    J 

"  "  iron,  "        chalybeate  water. 

Sulphate  of  lime  (gyfisum),      v       „  nent  hard  water. 

'  magnesia  {Epsom  salt)  J 

Chloride  of  sodium      {common  salt),        "        saline  water. 
Nitrate  of  potassium    (saltpeter), 
Sulphate  of  sodium      (Glauber  salt), 
Bicarbonate  of  sodium  (common  soda), 
Sulphate  of  alumina  with  the \/    r      \ 
Sulphate  potassium  or  sodium  J  ^ 
Silica,  making  silicious  water. 


-  making  alkaline  water. 


It  is  the  relatively  large  or  small  quantity  of  lime  or  magnesia 
contained  in  the  water  which  renders  it  hard  or  soft.  Soap  produces 
lather  in  soft  water ;  in  hard  water  it  does  not.  If  these  minerals 
are  present  as  carbonates,  they  may  be  removed  from  solution  by 
boiling ;  if  they  are  sulphates,  the  water  is  permanently  hard. 

Caverns,  Sink-holes,  and  Spring  Lakes. — Caverns, 
or  subterranean  tunnels  and  chambers,  are  formed  by  the 
prolonged  solution  and  abstraction  of  mineral  matter  by 
percolating  water.  In  some  limestone  districts,  owing  to 
the  solubility  of  this  rock,  such  caverns  are  often  many 
miles  in  extent.  Mammoth  Cave,  in  Kentucky,  and  the 
Luray  Caverns  of  Virginia  are  noted  instances.  By  the 
falling  in  of  the  roofs  of  caverns,  or  by  the  solution  of  the 
rock  along  the  vertical  joints  that  serve  as  channels  for  de- 
scending rain-water,  sink-holes  are  formed  —  such  as  occur 

P.  G.— 12. 


202  PHYSICAL    GEOGRAPHY. 

in  the  blue  grass  region  of  Kentucky.  The  surface  drain- 
age, creeks,  and  even  large  streams  may  disappear  in  sink- 
holes directly  underground,  where  they  greatly  hasten  the 
work  of  cave  formation.  The  falling  of  the  roof  of  a 
cavern,  by  obstructing  an  underground  stream,  might 
cause  the  water  to  rise  through  the  debris  and  form  a  lake 
in  the  sink-hole.  The  enormous  springs  of  Florida,  as 
Silver  Spring  and  Orange  Spring,  into  which  steamboats 
can  ascend,  as  well  as  many  large  spring  basins  in  other 
limestone  regions,  were  probably  formed  in  this  manner. 

Since  mineral  matter  in  solution  does  not  impair  the  clearness  of 
spring  water,  the  amount  abstracted  from  the  rocks  is  seldom  appre- 
ciated. Average  spring  water  contains  more  than  10g00ths  of  its 
weight  of  dissolved  mineral  matter.  The  springs  of  the  United 
States  east  of  the  Mississippi  are  almost  innumerable,  but  the  dis- 
charge of  water  from  only  900  of  them  has  been  measured :  these 
collectively  bring  to  the  surface  each  year  a  quantity  of  the  under- 
ground rock  equal  to  a  mass  io  feet  square  and  two  miles  long. 

Deposits   of    Springs    and    Percolating  Waters. — 

The  power  of  water  to  dissolve  most  minerals  increases 
with  its  temperature  and  the  amount  of  gases  it  contains. 
Percolating  water  at  great  depths,  therefore,  generally  dis- 
solves more  mineral  matter  than  it  can  hold  in  solution 
when  it  reaches  the  surface,  where  it  cools,  and,  being  re- 
lieved of  pressure,  much  of  its  carbonic  acid  gas  escapes 
to  the  atmosphere  or  is  absorbed  by  aquatic  plants  or 
mosses.  Hence,  deep-seated  springs  are  usually  sur- 
rounded by  a  deposit  of  the  minerals  with  which  the 
water  is  impregnated.  Sometimes  this  deposit  may  even 
form  large  hills;  sometimes  it  forms  a  mound  around  the 
spring,  over  the  sides  of  which  the  water  falls,  while  the 
spray,  evaporating  from  surrounding  objects,  leaves  them 
also  incrusted  with  a  mineral  deposit.  Percolating  water 
evaporating  on  the  sides  and  roof  of  limestone  caverns, 
leaves  the  walls  incrusted  with  carbonate  of  lime  in  beau- 


SPRINGS. 


203 


Fig.  84.— Scenes  in  Mammoth  Cave,  Ky. 

tiful  masses  of  crystals.  Water  slowly  evaporating  as  it 
drips  from  the  roof  of  caverns  to  the  floor  beneath  leaves 
a  deposit  on  both  places,  which  gradually  grows  down- 
ward from  the  roof  as  a  stalactite,  and  upward  from  the 
floor  as  a  stalagmite,  until  these  meet  and  form  one  con- 
tinuous column  of  stone. 

The  deposit  of  calcareous  springs,  or  travertine,  may  be  white, 
or,  if  iron  is  also  present  in  the  water,  it  may  be  yellow,  brown,  red- 
dish, or  beautifully  striped.  Chalybeate  or  iron  spring  deposits  vary 
from  bright  yellow  to  brown.  Sulphur  is  frequently  deposited  by 
springs  impregnated  with  sulphuretted  hydrogen, and  white  siliceous 
sinter,  by  hot  springs  in  volcanic  districts. 


204 


PHYSICAL    GEOGRAPHY. 


Land-slips. — Absorbed  water  lessens  the  cohesion  of 
most  rocks ;  it  renders  impermeable  clay  more  or  less 
plastic  and  slippery,  and  tends  to  soften  many  permeable 
limestones  and  sandstones.  When  saturated,  rocks  at 
some  depth  below  sloping  surfaces  may  thus  allow  the 
overlying  rocks  to  slide  downward  under  the  pressure  of 
their  own  weight,  especially  when  that  is  increased  by  the 
weight  of  an  unusually  large  quantity  of  percolating  water 
during  wet  weather.  Such  movements  are  called  land- 
slips (page  261). 


Fig.  85. 


Fig.  86. 


Small  land-slips  (Fig.  85)  in  which  the  soil  and  subsoil  to  a  depth 
f»f  a  few  feet  slip  a  short  distance  downward,  are  common  on  all  hill- 
sides, especially  where  the  subsoil  is  underlaid  by  strata  of  clay, 
as  is  the  case  in  the  vicinity  of  Cincinnati.  Large  land-slips  are 
generally  confined  to  hilly  or  mountainous  regions,  where  the  strata 
are  inclined  at  nearly  the  same  angle  as  the  surface.  In  such  local- 
ities serious  land-slips  are  not  uncommon.  Some  stratum,  as  AB 
(Fig.  86),  has  its  cohesion  weakened  by  moisture  until  it  is  not 
able  to  support  the  weight  of  the  overlying  mass  C,  which  suddenly 
starts  downward,  carrying  with  it  the  forests,  houses,  and  every  thing 
on  its  surface.  The  mass  overwhelms  whatever  it  meets,  and  may 
form  a  natural  embankment  across  the  valley  at  D.  By  obstructing 
the  flow  of  the  drainage,  such  an  embankment  may  cause  the  for- 
mation of  a  more  or  less  permanent  lake  on  its  upper  side.  Many 
mountain  lakes  have  thus  been  formed  by  land-slips. 


CHAPTER  XV. 


STREAMS. 


Then  the  channels  of  water  appeared,  and  the  foundations  of  the  world  were 
laid  bare.— Psalm  xviii  :  15. 

Streams  are  bodies  of  water  flowing  in  definite  chan- 
nels from  a  higher  to  a  lower  level  over  the  earth's  sur- 
face. The  water  in  streams  is  the  excess  of  the  rain-fall 
on  the  land  over  evaporation.  Streams  are  called  rills, 
brooks,  creeks,  and  rivers  as  they  increase  in  relative  size. 

Sources  and  Mouth. — The  beginning  of  a  stream,  at 
the  higher  level,  is  called  its  source.  The  source  of 
a  stream  is  generally  a  spring,  which,  it  has  been  seen,  is 
but  the  re-appearance  of  absorbed  rain-fall ;  but  the  source 
may  be  a  mass  of  melting  ice  or  snow,  a  lake,  a  swamp 
or  marsh,  or  simply  the  water  of  a  shower  that  flows  over 
the  surface  after  the  soil  is  completely  saturated.  The 
place  where  a  stream  joins  or  flows  into  a  larger  stream, 
a  lake,   or  the  sea,   is  called  its  month. 

General  Law  of  Streams. — Water,  when  free  to 
move  under  gravity,  always  flows  to  the  lowest  attainable 
level  and  by  the  steepest  path  it  can  find.  Therefore, 
streams  always  occupy  lines  of  depression,  or  valleys.  Hence, 
streams  generally  increase  in  size  as  they  advance  in  con- 
sequence of  the  constant  addition  of  water  from  the  sides 
of  the  valley.  This  water  collects  in  the  depressions  in 
the  valley  sides,  down  which  it  flows  as  minor  streams  or 
tributaries  to  the  main  stream  in  the  bottom  of  the  valley. 


206  PHYSICAL    GEOGRAPHY. 

Thus,  the  Ohio  and  Arkansas  rivers  drain  parts  of  the  valley- 
sides,  and  are  tributaries  to  the  great  Mississippi;  the  Wabash, 
Miami,  and  Licking  rivers  perform  the  same  office  in  the  smaller 
Ohio  valley,  and  are  tributaries  to  that  river;  the  Whitewater  and 
Mad  rivers  are  similarly  tributaries  to  the  Great  Miami ;  and  so  on 
down  to  the  smallest  streams,  whose  tributaries  are  mere  threads 
of  water,  hidden,  perhaps,  under  the  grass  or  fallen  leaves. 

Stream  Systems  and  Drainage  Basins. — A  stream, 
and  all  the  lesser  streams  that  contribute  water  to  it,  con- 
stitute collectively  a  stream  system.  The  whole  surface 
of  the  land  whose  inclination  is  such  that  it  contributes 
water  in  time  of  wet  weather  to  any  stream  of  a  system 
fa  called  the  drainage  basin,  or  simply  the  basin  of  the 
main  stream  of  that  system . 

Water-shed. — The  boundary  line  of  a  drainage  basin  is 
called  the  water-shed,  the  water  parting,  or  simply  the 
divide,  between  that  and  adjacent  basins.  The  location 
of  water-sheds  is  exactly  the  reverse  of  that  of  streams; 
they  always  occupy  lines  of  elevation.  The  crest  ot 
every  sharp  ridge  forms  a  water-shed,  but  the  top  of  an 
imperceptible  swell  in  an  apparently  level  prairie  is  also  a 
true  water-shed.  Hence,  a  water-shed  may  be  denned  as 
the  irregular  line  of  relatively  high  land  formed  by  the 
meeting  of  opposing  slopes,  whether  the  slopes  are  long 
or  short,  flat  or  steep. 

The  chart  (Fig.  87)  shows  the  main  water-shed,  the  drainage  basin, 
and  the  principal  streams  constituting  the  system  of  the  Mississippi 
River.  In  the  west  and  east  respectively  the  water-shed  generally 
follows  the  lofty  crests  of  different  ridges  of  the  Rocky  and  Appa- 
lachian mountain  systems,  but  in  each  locality  it  sometimes  crosses 
from  one  ridge  to  another,  following  the  highest  part  of  the  inter- 
vening valley.  In  crossing  from  one  mountain  system  to  the  other, 
the  water-shed  follows  the  line  which  is  continuously  the  highest 
across  the  intervening  low  country.  Near  the  head  of  Lake  Michi- 
gan this  great  water-shed,  which  divides  the  drainage  of  nearly  the 
whole  grand  division,  lies  in  the  apparently  level  prairies  scarcely 


STREAMS. 


207 


Fig.  87. 

600  feet  above  the  sea.     Each  stream  of  a  great  river  system  has  a 
minor  stream  system,  basin,  and  water-shed  of  its  own. 

Oceanic  and  Inland  Drainage  Basins. — The  almost 
continuous  highland  region  that  lies  near  the  convex  mar- 
gin of  the  continental  plateau  forms  the  main  water-shed 
of  the  land.  Thence,  the  surface  descends  by  long  and 
gentle  slopes  toward  the  Atlantic  and  its  great  arms,  but 
by  comparatively  short  and  steep  slopes  toward  the 
Pacific  and  Indian  oceans.  These  slopes  embrace  the 
drainage,  or  hydrographic,  basins  of  the  respective  oceans. 
There  are  areas  in  each  grand  division  where  the  rain-fall 
is  so  slight  in  comparison  with  the  evaporating  power  of 
the  air  that  all  the  streams  are  entirely  evaporated  before 
they  can  traverse  the  region.     These  regions  of  deficient 


208  PHYSICAL    GEOGRAPHY. 

rain-fall,  or  of  low  relative  humidity,  and  the  territory 
draining  into  them  are  called  inland  basins,  because  they 
contribute  no  streams  to  any  ocean.  By  far  the  largest 
area  of  the  land  lies  on  the  Atlantic  side  of  the  main 
water-shed.  Fully  one  half  of  the  land  on  the  globe  con- 
tributes its  drainage  to  the  Atlantic,  and  only  about  one 
eighth  to  the  Pacific  and  Indian  ocean  basins  respectively. 
The  inland  basins  collectively  cover  about  one  fourth  of 
the  land  surface. 

The  Discharge  of  Streams. — No  stream  discharges 
at  its  mouth  all  of  the  rain-fall  which  occurs  in  its  basin. 
In  traversing  the  basin,  the  streams  are  diminished  in 
volume  (i)  by  evaporation,  (2)  by  subterranean  channels 
leading  into  some  other  basin  or  to  submarine  outlets,  and 
(3)  by  chemical  change  of  the  water  into  some  other  sub- 
stance, either  in  the  soil,  in  plants,  or  in  animals.  The 
diminution  by  evaporation  is  vastly  greater  than  that  from 
all  other  causes.  The  proportion  of  rain-fall  discharged 
varies  greatly  in  different  basins,  depending  on  the  intri- 
cate local  conditions  which  occasion  the  disappearance  of 
water,  such  as  relative  humidity  of  the  air,  permeability 
of  the  surface  rocks,  character  of  the  region  with  respect 
to  vegetation,  etc.  No  great  basin  discharges  into  the  sea 
much  more  than  one  half  the  rain-fall  it  receives.  The 
Yukon,  the  Magdalena,  and  the  Rhine  discharge  about 
one  half;  the  Amazon  and  the  Mississippi  about  one  fifth ; 
and  the  Nile,  whose  lower  course  traverses  a  rainless 
region,  but  -gVtn  of  the  rain-fall  of  their  respective  basins. 
The  average  discharge  into  the  sea  from  all  streams  in  the 
world  is  estimated  to  be  only  one  fourth  to  one  fifth  of  the 
rain-fall  on  the  land.  This  small  proportion,  however, 
amounts  to  about  6,500  cubic  miles  annually — a  volume 
of  water  great  enough  to  cover  the  whole  United  States, 
including  Alaska,  to  a  uniform  depth  of  g}4  feet. 


(«>9) 


2IO  PHYSICAL    GEOGRAPHY. 

Relative  Size  of  Streams. — The  true  measure  of  the 
absolute  size  of  a  stream  or  stream  system  is  the  volume 
of  running  water  it  contains.  This  volume  changes  from 
day  to  day  and  from  season  to  season,  and  depends  upon  so 
many  factors,  that  its  determination  is  practically  impossi- 
ble. The  relative  or  comparative  size  of  great  stream 
systems  is  approximately  indicated  by  the  mean  annual 
volumes  of  rain-fall  occurring  in  their  respective  basins, 
which  depends  simply  upon  the  mean  depth  of  the  rain- 
fall and  the  area  of  the  basin.  The  opposite  table  indi- 
cates graphically  the  relative  sizes  of  the  thirty-three  great 
river  systems  determined  by  this  method. 

It  will  be  noticed  that  some  systems,  as  the  Amazon,  Kongo,  La 
Plata,  and  Nile,  owe  their  prominence  both  to  the  heavy  rain-fall  and 
to  the  great  area  of  their  basins.  In  others,  as  the  Mississippi  and 
the  Siberian  rivers,  less  than  the  average  rain-fall  is  compensated 
by  the  great  extent  of  their  basins;  while  in  still  others,  as  the 
Orinoco,  San  Francisco,  Irrawaddy,  and  especially  the  Magdalena, 
small  basins  are  compensated  by  exceptionally  heavy  rain-fall.  The 
discharge  at  the  mouth  of  a  system,  and  the  length  of  its  longest 
stream,  are  sometimes  used  as  indications  of  its  relative  size,  but  a 
large  system  may  lose  most  of  its  water  in  its  lower  course  and  dis- 
charge a  relatively  small  quantity  of  water,  while  a  long  stream  may 
be  shallower  and  have  fewer  and  shorter  tributaries  than  a  shorter 
stream ;  thus,  the  Nile,  though  three  times  as  long  as  the  Ohio-Alle- 
ghany, discharges  only  two  thirds  as  much  water,  while  the  Mis- 
sissippi-Missouri, the  longest  stream  in  the  world,  discharges  but 
little  more  than  one  fourth  as  much  water  as  the  Amazon. 

The  Longest  Rivers  in  the  World. 


Mississippi-Mo.,  4,192  miles. 

Nile     ....  4,018 

Yangtze-Kiang  3,156       " 

Amazon  .     ,    .  3,061       " 


Yenisei     .  .  2,950  miles. 

Amoor      .  .  2,919  " 

Kongo.     .  .  2,881  " 

Mackenzie,  .  2,866  " 


The  Largest  Mean  Annual  Discharges. 


Amazon   .     528  cubic  miles. 
Kongo       .     419     "         " 
La  Plata  .     189     " 


Mississippi,    .     145  cubic  miles. 
Yangtze-Kiang    125     "         " 
Orinoco     .     .     122     "        " 


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20  Volga 

21  Yukon 

22  Murray 

23  Saskatchewan 

24  Hoang  Ho 

26  Mackenzie 

27  Rio  Grande 

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(six) 


212 


PHYSICAL    GEOGRAPHY. 


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Fig.  88. 

Slope. — No  stream  has  a  regular  or  uniform  slope  from 
its  source  to  its  mouth.  If  any  stream  be  examined  at 
low  water,  it  will  be  found  to  present  a  succession  of 
short  but  relatively  steep  descents,  alternating  with  longer 
reaches  where  the  fall  is  much  less.  Now,  the  speed  or 
velocity  of  a  stream  depends  mainly  upon  the  inclination 
of  its  surface.  The  water  creeps  slowly  over  the  long,  flat 
reaches,  but  makes  the  shorter  and  steeper  descents  with 
a  rush,  forming  rapids  or  ripples  where  the  descent  is  an 
incline,  but  cataracts  or  cascades  if  the  descent  is  vertical. 
If  these  minor  irregularities  of  slope  be  disregarded,  and 
only  the  general  inclination  be  observed — as  in  the  profiles, 
(Fig.  88) — the  slope,  though  it  varies  in  different  parts  of 
the  course,  will  as  a  general  rule  be  found,  like  that  of 
mountain  sides,  to  decrease  in  steepness  as  it  is  descended. 
The  average  fall  per  mile  in  the  lower  course  of  five  of  the 
rivers  indicated  above  is  from  2  to  6  inches,  while  neat 
their  sources  it  varies  from  4^  to   120  feet  to   the  mile. 


STREAMS.  213 

Speed  of  Streams. — The  velocity  of  streams  is  gen- 
erally a  little  greater  just  below  the  surface,  than  at  the  sur- 
face or  nearer  the  bottom ;  and  is  greatest  near  the  middle 
of  the  stream.  A  stream  may  be  considered  as  composed  of 
a  number  of  layers  of  water  roughly  parallel  with  the  cross 
section  of  its  bed  (Fig.  89).  The  advance  of  each  layer  is 
somewhat  retarded   by       a 

friction.      The    bottom     ^^"(^     T"Zj~"'^0/ 
layer  a  is  pressed  upon  ^^^:^^z^:^^^^r 

by  the  greatest  weight  ^^^^^^^?^^ 

of  water,    and    moving 
upon  the  irregular  bed 

rock,  its  retardation  by  friction  is  greatest  and  it  moves 
slowest.  The  friction  of  each  successive  layer  above  a  is 
less  than  that  of  the  layer  upon  which  it  moves,  since  it  is 
pressed  upon  by  a  less  weight  of  water.  Hence,  if  the  line 
xy  represents  the  surface  of  the  stream,  the  layer  d  oc- 
cupying the  central  portion  of  the  surface  is  retarded  least 
by  friction,  and  therefore  flows  fastest.  If,  however,  the 
surface  is  raised  to  wv  during  a  freshet,  the  layer/,  being 
farther  from  the  bottom,  flows  still  faster. 

The  average  speed  of  an  ordinary  river  current  varies  from  less 
than  \l/2  miles  an  hour  at  low  water,  to  less  than  6  miles  at  high 
water,  while  exceptionally  rapid  torrents  probably  never  exceed  a 
speed  of  20  miles  an  hour.  The  Ohio  River  at  Cincinnati,  where 
its  fall  is  4  inches  to  the  mile,  has  a  mean  surface  speed  of  \x/% 
miles  an  hour  when  the  water  is  low  (6  feet  deep).  When  the 
water  is  high  (54  feet  deep)  the  average  current  is  nearly  6  miles  an 
hour ;  that  is,  it  is  6.35  miles  in  the  channel  and  5.85  miles  per  hour 
half-way  toward  either  bank.  The  Mississippi  River  at  Baton  Rouge, 
where  the  fall  is  3  inches  per  mile,  has  a  mean  speed  at  low  water 
of  \x/z  miles,  and  at  high  water  of  4  miles  an  hour. 

Variations  in  Volume. —  Owing  to  the  intermittent 
supply  of  rain  and  snow  water,  many  streams  are  subject 


514  Physical  geography. 

to  great  variations  in  volume.  Rains,  or  the  melting  of 
snow,  over  a  considerable  portion  of  any  drainage  basin  re- 
sult in  a  greater  or  less  rise  of  its  streams.  A  short, 
heavy  rain-fall,  or  the  rapid  melting  of  snow,  though 
yielding  a  comparatively  small  volume  of  water,  may,  on 
account  of  its  suddenness,  cause  a  greater  rise  at  a  given 
locality  than  a  greater  but  more  gradual  increase  of  vol- 
ume. An  exceptionally  great  or  rapid  increase  in  the 
volume  of  water  in  any  basin  may  cause  the  streams  to 
overflow  their  usual  banks  or  channels,  and  spread  over 
the  adjacent  lowlands,  producing  a  flood,  called  a  freshet 
in  small  streams. 

The  same  volume  of  water  causes  streams  to  rise  to  different 
heights  at  different  points  along  their  course,  depending  upon  the 
variations  in  the  widths  of  the  valley  and  the  slopes  of  the  stream. 
Where  the  valley  is  narrow,  the  same  volume  of  water  causes  the 
stream  to  rise  higher  than  where  the  water  can  spread  out  over  a 
greater  width  of  valley.  High  water  tends  to  make  the  surface  slope 
of  streams  uniform  by  increasing  the  slope  on  level  reaches  and  de- 
creasing it  at  rapids.  Thus,  the  greatest  known  flood  in  the  Ohio 
(188.4)  caused  a  rise  of  46  feet  above  the  "  Falls  "  (rapids)  at  Louis- 
ville, but  of  72  feet  three  miles  below,  at  the  foot  of  the  rapids. 

These  fluctuations  of  volume,  since  they  depend  en- 
tirely upon  the  weather  in  the  respective  drainage  basins, 
are  always  irregular  in  amount,  and  more  or  less  irregular 
in  time  of  occurrence.  Local  showers,  falling  on  compar- 
atively small  areas,  may  be  occasioned  by  many  purely 
local  and  temporary  conditions  of  the  atmosphere,  and 
therefore  occur  at  irregular  intervals ;  hence,  small  streams, 
whose  basins  lie  more  or  less  completely  within  the  small 
areas  of  these  local  rains,   fluctuate  irregularly. 

The  volume  of  water  which  flows  at  once  over  the 
surface  from  these  showers,  though  ample  to  swell  the 
smaller  streams,  is  seldom  sufficient  to  have  a  very  per- 
ceptible  effect   upon   the   main  streams  of  a  great  river 


STREAMS.  215 

system,  whose  basin  embraces  a  very  extensive  area. 
Enough  water  to  cause  a  marked  rise  in  such  streams  is 
only  supplied  by  wide-spread  or  lorlg-continued  rains  or 
by  the  melting  of  extensive  snow-fields.  Such  effects  are 
caused  chiefly  by  the  varying  amount  of  heat  received 
from  the  sun  at  different  seasons,  and  therefore  depend 
largely  upon  the  regular  movement  of  the  earth  in  its 
orbit.  Hence,  a  certain  corresponding  regularity  is  noticed 
in  the  recurrence  of  fluctuations  of  all  large  streams. 

The  Ganges  and  the  Nile  exhibit  special  regularity  in  time  of  re- 
currence of  low  and  high  water,  but  a  greater  or  less  degree  of  reg- 
ularity is  exhibited  in  this  respect  by  all  large  streams.  The  increas- 
ing heat  of  summer  not  only  melts  the  snows  on  the  lofty  Himalayas, 
but  causes  the  moist  monsoon  winds  to  blow  from  the  ocean ;  the 
resulting  rains  aid  the  snow-water  in  swelling  the  Ganges.  An 
annual  rise  begins  in  May  and  continues  till  September.  The  rise 
is  from  30  to  45  feet  at  Allahabad,  and  7  to  10  feet  at  Calcutta.  In 
the  same  manner  the  moist  spring  monsoons  deposit  excessive  rain- 
fall on  the  highland  of  eastern  Africa  from  Abyssinia  to  the  equator. 
Thus,  the  headwaters  of  the  Nile  begin  to  rise  annually  about  the 
1st  of  May,  but  it  takes  two  months  for  the  rise  to  reach  Cairo, 
where,  by  about  October  1st,  the  river  has  reached  a  height  of  be- 
tween 18  and  30  feet  above  its  June  level.  Throughout  the  United 
States  the  rains  are  more  uniformly  distributed  over  the  year,  but 
the  great  Rocky  Mountain  tributaries  to  the  Mississippi  are  gener- 
ally highest  through  June  and  July  on  account  of  the  snow-water 
from  their  elevated  sources.  The  snow  about  the  less  elevated 
sources  of  the  Ohio,  however,  melts  earlier  in  the  season,  and  Feb- 
ruary is  the  month  of  flood  in  that  river.  In  consequence  of  the 
different  times  of  flood  in  its  large  tributaries,  the  minor  fluctuations 
of  the  lower  Mississippi  are  somewhat  irregular,  but  it  is  always 
above  its  mean  level  from  January  1st  to  August  1st,  and  below  it 
during  the  rest  of  the  year.  The  usual  range  between  low  and  high 
water  in  the  Missouri  increases  from  about  6  feet  at  Fort  Benton, 
Mont.,  to  about  35  feet  at  its  mouth ;  in  the  Arkansas  from  10  feet 
at  Fort  Gibson,  Oklahoma,  to  45  feet  at  its  mouth.  The  usual  range 
of  the  Mississippi  above  Hannibal,  Mo.,  is  14  to  20  feet,  increasing 
to  50  feet  from  Cairo,  111.,  to  Red  River,  and  decreasing  thence  to 
nothing  at  its  mouth.    The  range  of  the  Ohio  is  about  50  feet 


CHAPTER  XVI. 

WORK   OF   STREAMS. 

The  waters  wear  the  stones;  thou  wxshest  away  the  things  which  grow  ou*  of 
the  dust  of  the  earth.— Job  xiv  ;  ig. 

Transportation — It  has  been  said  (page  181)  that  a 
process,  called  erosion,  is  constantly  at  work  over  the  sur- 
face of  the  land,  this  process  consisting  of  the  disintegra- 
tion and  removal  of  the  land  surface  particle  by  particle. 
The  weather  is  the  principal  cause  of  the  general  disinte- 
gration of  the  surface ;  streams  are  the  principal  means  of 
the  removal  of  the  disintegrated  material.  During  its 
transport,  each  particle  becomes  a  tool  with  which  the 
stream  powerfully  and  rapidly  disintegrates  and  wears 
away  its  bed,  even  if  the  bed  consists  of  the  hardest  rock. 
This  method  of  disintegration,  which  takes  place  only  in 
the  beds  of  running  streams,  is  called  corrasion  to  distin- 
guish it  from  the  more  general  disintegration  of  the  whole 
surface  of  the  land  by  the  weather.  Streams  transport 
rocky  material  in  three  ways:  (i)  in  solution,  (2)  in  sus- 
pension, and  (3)  by  rolling  or  pushing  it  forward  along  the 
stream  bed. 

In  solution. — While  rocky  material  is  in  solution  or 
dissolved  in  water,  it  is,  strictly  speaking,  no  longer  rock, 
its  molecules  having  been  separated  to  form  a  constituent 
part  of  the  water.  When  in  this  form  the  rocky  material 
accompanies  the  water  in  all  its  movements  and  to  any 
distance,  until  some  change  of  temperature  or  pressure 
renders  the  water  unable  to  hold  it  *\11,  when  a  portion  of 

(ai6) 


WORK   OF   STREAMS.  217 

the  dissolved  matter  is  precipitated  into  its  true  rocky  form 
again. 

In  suspension. — The  transportation  of  rock  particles 
in  suspension  is  entirely  different,  and  is  simply  mechan- 
ical, depending  upon  the  swiftness  of  the  current  and  the 
size  of  the  particles.  Larger  or  smaller  rock  or  soil  par- 
ticles are  constantly  finding  their  way  into  streams  by 
their  own  weight  or  by  the  force  of  the  winds,  but  chiefly 
through  the  wash  of  successive  rains.  Once  in  the  stream 
their  more  rapid  journey  begins.  The  rock  particles, 
being  heavier  than  water,  have  a  tendency  to  sink,  and 
would  go  straight  to  the  bottom  if  the  water  were  still ; 
but  in  its  general  advance  over  the  irregularities  of  its 
bed,  an  intricate  system  of  minor  currents  is  set  up  in  the 
body  of  the  stream,  which  move  upward  and  sideways, 
and  occasion  the  "  boiling"  places,  waves,  and  whirlpools 
always  seen  on  the  surface  of  rapid  streams.  These  minor 
currents  prevent  the  sinking  of  the  finer  rock  particles, 
which  are  therefore  carried  along  by  the  main  current  in 
suspension.  Should  the  main  current  increase  in  velocity, 
the  force  of  the  minor  currents  increases,  and  larger  par- 
ticles can  be  held  in  suspension ;  should  the  speed  of 
the  main  current  become  slower,  the  minor  currents  de- 
crease in  force,  and  the  larger  particles  in  suspension  sink 
to  the  bottom.  It  is  the  material  in  suspension  that 
causes  the  muddiness  or  turbidity  of  stream  water;  and 
the  general  increase  in  its  turbidity  after  rains  is  occa- 
sioned both  by  the  large  amount  of  fine  soil  particles 
washed  in  by  the  rain,  and  by  the  ability  of  the  stream  to 
carry  along  more  and  larger  particles  in  suspension  when 
its  volume  and  velocity  are  increased  by  the  shower. 

The  capacity  of  running  water  for  material  in 
suspension  increases  very  rapidly  as  its  speed  increases; 
to  double  its  speed  would  increase  its  capacity  about  64 


21 8  PHYSICAL   GEOGRAPHY. 

times.  Hence,  very  much  more  material  is  transported 
by  streams  in  times  of  flood,  or  high  water,  than  when  the 
water  is  low. 

If  the  material  is  fine  enough  to  be  held  in  suspension,  the  trans- 
porting capacity  of  water  flowing  at  any  speed  is  very  great.  If  this 
capacity  were  reached,  the  stream  would  appear  as  a  mass  of  very 
fine  mud  or  "  quicksand,"  just  liquid  enough  to  flow,  and  the  water 
would  form  but  one  fourth  of  its  weight ;  that  is,  out  of  every  five 
cubic  feet  of  the  liquid  mass,  three  cubic  feet  would  be  solid  par- 
ticles. It  is  very  seldom  that  enough  soil  particles,  small  enough  to 
be  held  in  suspension,  are  washed  into  a  stream  at  one  time  to  fill  it 
nearly  to  the  limit  of  its  transporting  capacity ;  but  such  mud-  or 
sand- streams  are  occasionally  encountered. 

Material    pushed   forward    on    the    stream    bed. — 

Innumerable  particles  and  rock  fragments,  too  large  to  be 
held  in  suspension,  are  yet  small  enough  to  be  rolled  for- 
ward along  the  bottom  of  streams  with  great  force  by  the 
main  current.  It  is  to  the  attrition  of  such  fragments 
and  of  those  in  suspension,  that  the  wearing  away  or 
deepening  of  stream  beds  is  chiefly  due.  The  size  of 
these  fragments,  the  force  with  which  they  advance,  and 
hence  the  amount  of  deepening  of  the  stream  bed,  or 
corrasion,  increase  very  rapidly  with  the  speed  of  the  main 
current.  The  deepening  of  any  stream  bed  by  corrasion 
of  course  increases  the  steepness  of  slope  of  its  valley 
sides;  hence,  the  speed  and  corrasive  power  of  all  its 
tributary  streams  are  increased,  and  this  increases  the 
slope,  speed,  and  corrasion  of  all  streams  flowing  into 
these  tributaries.  Therefore,  the  deepening  of  any  stream 
bed  increases  the  amount  of  disintegration  and  transporta^ 
tion — that  is,  of  erosion — over  its  entire  basin. 

Sedimentation. — Wherever  the  speed  of  a  current  is 
checked  from  any  cause,  the  water  is  no  longer  able  to 
hold  the  larger  particles  in  suspension;  they  therefore 
settle  to  the  bottom  to  be  either  rolled  along  or  left  be- 


WORK    OF    STREAMS.  219 

hind  as  sediment,  according  to  the  force  yet  remaining  in 
the  main  current.  Should  this  current  be  further  checked, 
the  water  becomes  clearer  as  still  smaller  particles  in  sus- 
pension are  deposited  upon  the  bottom,  until,  upon  the 
cessation  of  all  currents,  the  water  would  become  per- 
fectly clear  as  the  smallest  particles  in  suspension  are 
gradually  deposited. 

The  amount  of  material  transported  varies  greatly 
in  different  streams  according  to  their  slopes  and  the  char- 
acter of  their  basins,  and  in  any  one  stream  it  varies  greatly 
with  the  stage  of  water.  Many  calculations  on  different 
rivers  indicate  that  streams  on  the  average  transport  about 
Y^fg-g-ths  *  of  their  weight  of  mineral  matter.  That  is 
to  say,  the  rivers  of  the  world,  in  the  aggregate,  trans- 
port each  year  from  the  land  to  the  sea  enough  rocky 
material  to  make  a  sharp  crested  range  of  mountains 
1,000  feet  high,  a  mile  wide  at  the  base,  and  30  miles 
long. 

The  quantity  of  material  in  suspension  atone  discharged  annually 
by  the  Mississippi  would  make  a  range  of  hills  500  feet  high,  half  a 
mile  wide  at  base,  and  over  a  mile  long.  The  Ganges  discharges 
annually  about  the  same  amount  of  matter  in  suspension,  while  the 
suspended  matter  which  the  little  Rhone  discharges  annually  into 
the  Mediterranean  would  make  a  pyramid  a  mile  square  at  the  base 
and  230  feet  high. 

Formation  of  Valleys. — By  thus  removing  the  par- 
ticles disintegrated  by  atmospheric  agencies,  and  by  the 
attrition  of  these  particles  upon  the  stream  bed,  the 
streams  themselves,  during  the  long  ages  of  the  past, 
have  hollowed  out  and  formed  the  valleys  in  which  they 
flow,  and  the  same   processes   are   to-day  modifying   the 

*In  suspension,  .000558 — T.  M.  Reade,  Am.  Jour.  Science,  1885,  page  298. 

In  solution,  .000186 — J.  Murray,  Sc.  Geog.  Mag.,  1887,  page  76. 

Rolled  along  bottom,   .000067 — Hump,  and  Abbott,  Hyd.  Miss.  Riv.,  page  148. 

Total,  .000811 

P  G.-X3. 


220 


PHYSICAL   GEOGRAPHY. 


shape  of  every  foot  of  the  land.  The  variety  in  the 
slopes  and  shapes  of  valleys  results  from  the  varying  rate 
of  corrasion  and  weathering  in  different  regions  as  deter- 
mined by  slope,  climate,  and  hardness  of  the  rocky 
material  composing  the  earth's  surface. 

The  Curve  of  Erosion. — In  spite  of  these  causes  of 
variation,  the  general  slopes  of  different  parts  of  the  land, 
and  of  different  valley  bottoms  in  particular,  have  a  rough 
resemblance  in  becoming  flatter  as  they  are  descended. 
(See  diagrams,  pages  165  and  212.)  This  arrangement 
of  slopes  results  from  the  invariable  action  of  running 
water  and  the  peculiar  curve  which  it  produces  in  the 
general  slope  may  therefore  be  called  the  curve  of  corra- 
sion or  erosion. 

The  surface  of  the  land  is  a  succession  of  steep  and  gentle  slopes. 
All  parts  of  it  are,  constantly  or  intermittently,  subjected  to  the  action 
of  running  water ; — the  beds  of  permanent  streams,  constantly ;  and 
other  parts  of  the  land,  during  and  immediately  after  rains.     The 

stream  or  rain-water  flowing 
swiftly  down  the  steep  slopes 
corrades  more  material  than  its 
slower  current  is  able  to  trans- 
port over  the  flatter  slope  below ; 
hence,  a  deposit  is  formed 
against  the  bottom  of  the  steep 
descents  as  at  a,  a,  (Fig.  90). 
Subsequent  action  of  the  same 
kind  causes,  for  similar  reasons,  further  deposits  at  b  and  c.  Thus, 
the  profile  of  the  slope  gradually  acquires  the  form  of  a  succession 
of  curves  of  erosion.  But  the  deposits  are  formed  of  material  cor- 
raded  from  the  higher  parts  of  the  slope  d,  d,  which  are  thus  flat- 
tened as  indicated  by  the  dotted  lines.  When  this  part  of  the  slope 
becomes  as  flat  as  the  surface  of  the  deposit  at  b,  the  checking  of 
the  current  and  the  settling  of  sediment  ceases,  while  corrasion 
begins  to  cut  away  the  deposit  at  that  point.  Prolonged  action  of 
this  kind  gradually  wears  away  all  irregularities,  and  reduces  the 
slope  to  a  single  curve  of  erosion  (Fig.  91).  The  constant  action 
and  greater  volume  of  water  on  stream  beds  has  frequently  reduced 


Fig.  90. 


WORK    OF    STREAMS. 


221 


their  general  profile  to  a  single  curve  from  mouth  to  source;  but 
in  many  streams  the  reduction  has  not  yet  advanced  so  far,  and 
two  or  more  curves  can  be 
distinguished  in  the  general 
profile  of  the  stream,  as 
in  the  Colorado  and  the 
Nile  (Fig.  88).  The  general 
surface  of  the  land,  acted 
upon  by  smaller  volumes  of 
water,  and  only  at  intervals, 
is  reduced  more  slowly,  and  lg'  9I* 

its  profile,  except  in  its  more  general  features,  presents  a  long  suc- 
cession of  curves  of  erosion.  The  deposit  of  corraded  material  at 
the  place  where  a  current  of  water  is  checked  by  encountering  a 
gentle   slope  occasions  the   familiar  alluvial  cones,  or  fan-shaped 

heaps,  which  invariably 
occur  where  swift  mount- 
ain streams  or  the  com- 
mon wet  weather  gullies 
of  steep  hillsides  encounter 
the  more  gently  sloping 
plain. 

The  steepness  of 
the  sides  of  valleys 
depends  upon  the  rel- 

Fig.  92.— Alluvial  Cones  in  Utah.  .•  'j-.  r    ±x. 

6  *  ative    rapidity   of  the 

corrasion  of  the  stream  bed  at  the  valley  bottom,  and  the 
more  general  weathering  of  its  sides  by  rain,  frost,  etc. 
Where  corrasion  is  the  more  rapid,  the  valley  is  deepened 
faster  than  it  is  widened,  and  its  sides  are  steep,  giving  it 
more  or  less  the  shape  of  a  V.  When  weathering  is  the 
more  rapid,  the  valley  is  widened  faster  than  it  is  deep- 
ened, its  sides  become  flatter  and  lower,  and  its  cross  sec- 
tion is  more  basin-shaped.  The  general  rapidity  of 
weathering  is  usually  about  the  same  in  the  same  material 
in  all  parts  of  small  basins,  but  the  corrasive  power  of  the 
stream  decreases  rapidly  as  it  advances  down  the  succes- 
sively gentler  slopes  of  its  bed.     Hence,  as  a  general  rule, 


222  PHYSICAL   GEOGRAPHY. 

valleys  are  narrow  and  relatively  deep  in  the  upper  course 
of  streams,  but  gradually  become  wider  and  relatively 
shallower  as  the  stream  is  descended,  and  in  the  lower 
course  of  large  streams  may  become  so  wide  and  flat  as 
to  lose  entirely  all  visible  side  slope  and  become  practi- 
cally plains.  Thus,  the  lower  Mississippi  valley,  from 
Cairo  to  the  Gulf,  is  a  gently  sloping  plain  varying  in 
width  from  20  to  70  miles. 

The  rapidity  of  weathering  on  the  valley  sides  is  not 
always  the  same,  however,  throughout  the  same  drainage 
basin ;  it  varies  considerably  with  the  amount  of  rain-fall. 
If  part  of  the  course  of  a  stream  traverses  a  region  of 
either  very  heavy  or  very  light  rain-fall,  the  effect  is  im- 
pressed on  the  shape  of  its  valley.  Heavy  rain-fall  in- 
creases the  rate  of  weathering  of  the  valley  sides  and  of 
the  adjacent  upland,  and  a  proportionately  wider  and 
shallower  valley  is  the  result.  Light  rain-fall  has  the  op- 
posite effect,  and  favors  the  formation  of  deep,  narrow 
valleys,  with  steep  side-slopes.  Thus,  the  lower  valley 
of  the  Nile  has  a  very  gentle  slope,  and  corrasion  is  cor 
respondingly  slow ;  but  it  lies  in  so  dry  a  region  that  the 
rate  of  weathering  is  still  slower,  and  a  comparatively 
deep,  narrow  valley  is  produced.  When  the  rain-fall  is 
very  slight  and  the  slope  of  the  streams  is  very  great,  a 
cafwn,  or  valley  of  exceptional  narrowness  in  proportion  to 
its  depth,  is  formed.  Noted  examples  of  this  are  afforded 
by  the  Colorado,  Virgin,  and  many  other  streams  of  the 
Rocky  Mountain  plateau  region. 

The  character  of  the  material  has  an  important  influ- 
ence upon  the  general  transverse  shape  of  valleys.  Thus, 
the  streams  which  traverse  the  Great  Plains,  though  they 
have  a  steep  slope  and  traverse  a  region  of  scant  rain-fall, 
produce  valleys  so  wide  and  shallow  as  scarcely  to  merit 
the  name  valley.     This  is  because  the  rock  of  the  region 


WORK    OF   STREAMS. 


223 


Fig.  93.— Grand  Canon  ot  the  Colorado,  at  Toroweap. 


weathers  so  rapidly  that  its  surface  is  always  covered  with 
a  great  depth  of  sand  and  soil  particles  which  slide  into 
the  streams  from  the  sides  as  fast  as  other  particles  are  re- 
moved from  the  stream  bed  by  the  current.  Thus,  the 
stream  becomes  overloaded  with  sediment,  and  maintains 
a  deposit  of  sand  upon  its  bed  which  it  can  not  remove. 
The  bed  is  thereby  protected  from  corrasion;  hence,  the 


21\  PHYSICAL  GEOGRAPHY. 

valleys    are    constantly    getting    wider    without    getting 
deeper. 

The  rivers  which  flow  from  the  Rocky  Mountains  across  the 
Great  Plains,  like  the  Arkansas  and  the  Platte,  have  unusually  steep 
slopes,  being  about  as  steep  as  the  Colorado.  But,  after  leaving  the 
mountains,  they  cut  no  canons  or  deep  valleys,  while  the  Colorado 
has  cut  profound  ones.  The  difference  in  the  two  cases  is  due  to 
the  fact  that  the  river  troughs  of  the  Great  Plains  are  deeply  buried 
in  sand,  the  waters  of  the  rivers  being  loaded  to  their  utmost 
capacity,  while  the  Colorado  is  able  to  transport  more  sediment  than 
it  receives.  The  rocks  in  its  trough  are  to  a  great  extent  bare  owing 
to  the  scouring  action  of  the  material  in  suspension,  and  the  channel 
is  continuously  deepened. 

Variety  in  the  Character  of  Material. — When  hard 
strata  alternate  with  soft  ones,  the  sides  of  a  valley  form  a 
series  of  steep  and  flat  slopes,  the  steep  slopes  occurring 
in  the  hard  strata.  The  rapid  erosion  of 
some  soft  strata  frequently  undermines 
the  edges  of  a  hard  overlying  stratum. 
strata  £  '  j  - 4  j±  line  of  overhanging  cliffs  along  the 
valley  side  is  thus  formed  (Fig.  94).  Frag- 
ments of  the  cliff  often  fall  from  their 
own  unsupported  weight,  aided  by  the 
prying  action  of  freezing  water  in  the 
joints  (page  13).  If  such  fragments 
collect  faster  than  erosion  can  remove 
them,  they  gradually  form  a  talus,  which  may  cover  and 
for  a  time  protect  the  softer  strata  from  further  erosion. 

Cataracts  and  Cascades. — If  the  main  or  a  tributary 
stream  in  a  valley  whose  sides  contain  lines  of  cliff  be 
ascended  until  the  stream  bed  reaches  the  foot  of  the 
cliff,  a  water-fall  is  encountered.  It  may  be  a  cataract, 
called  a  cascade  in  small  streams,  or  simply  rapids,  accord- 
ing to  circumstances.  If  the  strata  are  nearly  horizontal, 
and  the  stream  clear  and  large  enough  to  reduce  and  carry 


WORK    OF    STREAMS.  225 

away  the  rock  fragments  about  as  fast  as  they  fall,  the 
overhanging  form  of  the  cliff  is  constantly  maintained. 
The  stream  leaps  over  this  as  a  cataract,  leaving  a  space 
under  the  hard  stratum  and  behind  the  falling  water  which 
is  constantly  filled  with  spray,  and  into  which  people  can 
frequently  enter  from  the  sides.  The  occasional  but  con- 
tinued detachment  and  fall  of  fragments  from  the  over- 
hanging cliff,  causes  a  constant  recession  of  the  cataract 
up  stream. 

The  cataract  of  Niagara,  midway  between  lakes  Erie  and  Ontario, 
is  about  165  feet  high.  Though  by  no  means  the  highest,  it  is  prob- 
ably the  grandest  cataract  in  the  world  on  account  of  its  great 
volume  of  water.  Immediately  above  the  fall  the  Niagara  River  is 
almost  a  mile  wide.  It  flows  over  the  brink  with  an  average  depth  of 
about  four  feet,  and  a  greatest  depth  of  perhaps  twenty  feet.  Enough 
water  flows  over  every  twenty -four  hours  to  make  a  lake  a  mile 
square  and  821  feet  deep.  Below  the  fall  the  river  occupies  a  nar- 
row valley,  or  canon,  which  gradually  increases  from  200  to  300  feet 
deep.  The  canon  is  so  narrow  that  in  it  the  river  has  only  one  eighth 
to  one  fourth  of  its  former  width.  The  stratum  of  hard  stone  that  forms 
the  brink  of  the  cataract  outcrops  along  the  top  of  the  canon,  form- 
ing a  line  of  cliffs,  beneath  which  a  talus  of  fragments  slopes  steeply 
down  to  the  water's  edge.  Seven  miles  below  the  falls  these  cliffs 
turn  sharply  away  from  the  stream  to  right  and  left,  and  the  river 
flows  thence  through  a  low,  open  country  to  Lake  Ontario.  The 
place  where  the  cliffs  turn  away  from  the  stream  undoubtedly  marks 
the  original  position  of  the  cataract,  while  the  seven  miles  of  canon 
is  the  amount  the  fall  has  receded  up  stream  from  the  constant  un- 
dermining and  breaking  away  of  the  hard  stratum.  Judging  from  the 
present  rate  of  recession  of  the  fall,  about  three  feet  a  year,  it  has 
required  12,320  years  for  the  excavation  of  the  canon.  It  has  prob- 
ably not  required  quite  so  long  a  time,  however,  for  at  present  a 
large  portion  of  the  river — the  American  Fall — falls  into  the  side  of 
the  canon  and  is  consequently  engaged  in  increasing  its  width  and 
not  its  length.  The  canon  is  so  narrow  because  the  talus  of  hard 
rock  fragments  from  the  cliffs  on  its  side  slopes  forms  a  protecting 
layer  over  the  soft  strata  beneath,  and  thus  prevents  to  a  great 
extent  the  undermining  and  downfall  of  the  cliffs. 


226  PHYSICAL    GEOGRAPHY. 

Rapids,  or  a  series  of  very  low  falls,  are  generally 
formed  instead  of  a  single  cataract  where  the  strata  have 
a  very  steep  dip,  or  when  the  stream  is  muddy,  or  is  not 
sufficiently  powerful  to  prevent  the  formation  of  a  talus 
under  the  edge  of  the  horizontal  strata;  for  in  these 
cases  the  form  of  an  overhanging  cliff  can  not  be  main- 
tained, and  the  water  simply  rushes  down  a  steep  broken 
incline.  Rapids  are  also  formed  by  many  other  obstacles 
to  the  uniform  descent  of  streams. 

Since  within  certain  limits  the  corrading  power  of  a  stream  in- 
creases with  the  amount  of  sediment  it  carries,  muddy  streams  may- 
wear  away  the  hard  stratum  forming  the  brink  of  a  water-fall  faster 
than  the  weather  erodes  the  soft  strata  beneath,  and  thus  prevent 
the  formation  of  an  overhanging  cliff  and  a  cataract,  the  water  sim- 
ply descending  a  steep  incline  as  a  rapid.  Indeed,  cataracts  are 
almost  invariably  confined  to  streams  carrying  clear  water,  such  as 
those  issuing  from  lakes.  Most  of  the  conditions  favoring  the  for- 
mation of  great  cataracts  exist  along  the  steep  course  of  the  Colo- 
rado River,  and  the  fact  that  only  rapids  occur  is  believed  to  be  due 
to  the  great  corrading  power  which  the  large  amount  of  sediment 
carried  gives  to  its  current.  It  is  believed  that  the  noble  cataract  at 
Niagara  would  be  quickly  reduced  to  a  simple  rapid  were  the  water 
of  the  river  very  muddy  instead  of  being  very  clear  as  it  is. 

Immense  age  or  permanence  is  often  suggested  by 
reference  to  hills  or  mountains, — such  sayings  as  "old  as 
the  hills,"  "the  everlasting  mountains,"  etc.,  being  com- 
mon. But  the  position  of  valleys,  which  are  every-where 
and  almost  invariably  the  work  of  running  water,  proves 
that  in  very  many  instances  the  courses  of  the  larger 
streams  are  older  than  the  bordering  hills  and  mount- 
ains. 

Instances  of  this  are  numerous  in  most  regions,  but  are  specially 
conspicuous  in  mountain  districts.  In  the  Appalachian  region  of 
New  York,  Pennsylvania,  and  Virginia  the  numerous  parallel 
mountain  ridges  seem  to  have  influenced  but  little  the  course  of  the 
larger  streams,  which  flow  directly  through  the  various  ridges  one 
after  another  in  a  succession  of  deep,  narrow  gorges  or  water  gaps, 


WORK    OF    STREAMS.  227 

such  as  the  Highland  gorge  of  the  Hudson,  the  Delaware  Water 
Gap,  the  Susquehanna  Water  Gap  above  Harrisburg,  the  Harpers 
Ferry  gorge  on  the  Potomac,  etc.  This  indicates  that  the  general 
course  of  these  streams  was  established  before  the  present  mountains 
existed,  and  was  maintained  by  the  constant  corrasion  of  the  stream 
bed.  Corrasion  thus  cuts  the  notches  or  gaps  in  the  several  ridges  as 
erosion  slowly  hollows  out  the  valleys  between  them. 

Deltas. — Upon  entering  a  body  of  water  with  little  or 
no  current,  as  a  lake,  a  stream  deposits  its  sediment,  pro- 
ducing a  submerged  alluvial  cone,  which  may  slowly  rise 
to  the  surface  of  the  water,  and  be  converted  into  a  fan- 
shaped  area  of  low,  marshy  land.  The  stream  generally 
traverses  this  new-made  land  in  several  radiating  channels, 
and  by  the  continued  deposit  at  the  several  mouths  causes 
the  land  constantly  to  advance  further  into  the  lake.  The 
extensive  deposit  of  this  kind,  accumulating  at  the  mouth 
of  the  Nile  is  exceptionally  regular  in  shape,  resembling 
the  Greek  letter  delta  (a),  and  from  it  the  name  delta  has 
come  to  be  applied  to  all  such  formations.  All  streams 
flowing  into  the  ocean  would  form  deltas  but  for  currents 
strong  enough  to  remove  the  sediment,  or  subsidence  of 
the  earth's  crust  rapid  enough  to  absorb  it,  as  fast  as  it 
is  deposited  at  the  mouth  of  the  stream.  Deltas  are  com- 
mon only  in  lakes  and  nearly  land-locked  seas,  for  these 
being  nearly  tideless,  are  less  likely  to  have  strong  cur- 
rents ;  such  are  the  Mediterranean,  the  Gulf  of  Mexico, 
the  Arctic  Ocean,  the  North  Sea,  the  east  Asia  seas,  etc. 

The  dividing  of  the  main  stream  into  the  radiating  branches 
which  gives  the  peculiar  form  to  the  delta  is  the  result  of  the  varying 
action  of  the  stream  at  low  and  high  stages  of  water.  Throughout 
its  lower  course,  where  the  slope  is  very  slight,  the  stream  at  low 
water  occupies  a  contracted  channel,  and  the  current  is  just  about 
able  to  move  along  the  load  of  sediment.  At  high  water,  the  stream 
spreads  out  over  the  whole  valley  bottom,  the  low  water  channel 
marking  the  deepest  water  and  swiftest  current,  while  on  each  side 
of  the  channel  the  current  is  much  slower,  and  a  great  deal  of  sedi- 


THE   DELTA 

and  the 

ALLUVIAL  BOTTOM 

gaeo  of  the  lower 

Mississippi  river 

and  the 

DELTA  or  the  NILE 

Scale . 


Above  High  Water  level 
Below  "  " 


JSBURG 

3fE>ST    GREENWICH  ?i°  ~  .    „      W  I  Pf     DPI   TA 

Mouth 


V    VQNGrrygE    W»T    FROM    GREENW.CH 


WORK    OF    STREAMS. 


229 


{Heights     exaggerated     500     times) 
Fig.  95.— Profiles  across  the  valley  bottom  of  the  Mississippi  Ri 


ment  is  deposited  and  left  as  a  layer  of  mud  when  the  water  sub- 
sides. Now,  this  deposit  is  greatest  on  the  banks  of  the  low  water 
channel,  where  the  rapid  current  suddenly  changes  to  a  slow  one ; 
these  banks  are  thus  raised  higher  than  the  valley  bottom  farther 
away  from  the  low  water  channel.  The  banks  continue  to  be 
raised  in  this  manner  by  successive  floods  until  they  become  so  high 
that  the  weight  of  the  stream  when  "  bank  full "  bursts  the  bank  at 
some  weak  point,  thus  causing  a  "  crevasse,"  through  which  part  of 
the  water  in  the  main  stream  drains  off  into  the  lower  land  and 
follows  down  the  side  of  the  valley  bottom.  When  this  occurs  at 
a  considerable  distance  from  the  mouth,  the  side  stream,  after  a 
longer  or  shorter  course,  as  a  bayou,  generally  finds  its  way  back 
into  the  main  low  water  channel  again;  but  when  crevasses  occur 
near  the  mouth  of  the  stream,  the  bayous  form  independent  mouths, 
and  by  the  corrasion  of  the  soft  layers  of  sediment  forming  their 
beds,  may  eventually  increase  in  width  and  depth  until  they  rival  or 
exceed  the  former  main  stream  in  volume.  The  Atchafalaya  Bayou 
of  the  Mississippi  is  the  highest  one  having  an  independent  mouth, 
and  its  divergence,  at  the  mouth  of  Red  River,  is  therefore  called 
the  head  of  the  Mississippi  delta. 

Estuaries. — When  a  coast  region  is  sinking  with  rela- 
tive rapidity  the  streams  are  apt  to  empty  into  deep  and 
narrow  bays,  fiords  or  estuaries,  formed  by  the  submerg- 
ence of  the  lower  part  of  the  stream  valley.  Chesapeake 
and  Delaware  bays  and  the  indentations  of  the  Maine 
coast  are  such  submerged  valleys. 


230 


PHYSICAL   GEOGRAPHY. 


Course. — As  a  rule,  the  path  of  a  stream  becomes 
more  devious  as  the  stream  is  descended,  because  the 
declivity  and  corrasive  power  usually  decrease  in  that 
direction.  A  mass  of  relatively  hard  material  in  the  bed, 
operates  to  deflect  the  stream  toward  the  side  composed 
of  softer  material.  A  small  but  steep  tributary  generally 
tends  to  deflect  the  main  stream  toward  the  opposite  side 
of  its  valley,  for  the  tributary,  being  swift,  brings  down 
particles  which  the  more  gentle  current  of  the  main  stream 
can  not  carry.  A  delta-like  deposit  or  bar,  therefore,  ad- 
vances into  the  main  stream  and  forces  its  current  against 
the  opposite  bank,  which  is  rapidly  corraded  into  a  loop- 
like bend.  A  large  tributary,  during  its  floods,  may  in  this 
way  deposit  material  entirely  across  the  main  stream, 
whose  waters  are  thus  dammed  back  into  a  long,  deep 
pool,  while  they  flow  over  the  deposit  as  a  shallow  rapid. 
Whenever  a  stream  is  deflected  from  a  straight  course, 
the  current  tends  to  increase  the  bend. 

Thus,  in  any  bend  of  a  stream,  as  BD  (Fig.  96),  the  inertia 
of  the  current  causes  it  to  follow  the  course  of  the  dotted  line; 

hence,  the  banks  at  B,  C,  and  D 
are  corraded  fastest,  while  sedi- 
mentation frequently  takes  place 
at  F%  E,  and  G,  and  sand-bars, 
beaches,  or  mud  flats  advance 
into  the  river  as  the  opposite 
bank  recedes.  Hence,  the  channel  containing  the  deepest  and 
swiftest  water  is  always  found  close  to  the  concave  bank  of  a  stream. 
The  effect  of  this  is  most  marked  in  the  lower  course  of  streams 
where  the  banks  are  composed  of  soft  sediment.  Figures  X,  Y,  and 
Z  indicate  progressive  states  of  a  bend  in  such  places.  In  Fthe 
narrow  neck  of  the  loop  has  been  cut  across.  The  descent  through 
this  short  "cut-off,"  being  steeper  than  it  is  around  the  loop,  the  cut- 
off rapidly  increases  in  depth  and  width  by  corrasion  until  it  be- 
comes the  main  channel.  The  ends  of  the  loop  are  soon  filled  with 
sediment,  and  the  crescent  shaped  lake  (Fig.  Z)  alone  remains  to 
mark  the  former  site  of  the  river. 


Fig.  96. 


CHAPTER  XVII. 

GLACIERS    AND    LAKES. 

Hast  thmi  entered  into  the  treasures  of  the  snow?— Job  xxxviii  :  22. 
Ye  shall  not  see  wind,  neither  shall  ye  see  rain;  yet  that  valley  shall  be  filltd 
•with  water,  that  ye  may  drink.— W  Kings  hi  ;  17. 

Glaciers. —Wherever  more  snow  falls  in  winter  than  is 
melted  in  summer,  the  snow  tends  to  accumulate  on  the 
ground  and  to  move  down  the  slopes.  Dry  and  powdery 
at  first,  the  snow,  in  passing  to  lower  levels,  gradually  be- 
comes compacted,  by  the  accumulating  weight  above  and 
the  freezing  of  percolating  water  from  the  melting  of  the 
surface  snow,  into  a  white,  granular  mass  called  neve. 
At  greater  depths  this  mass  is  compressed  into  more  or 
less  transparent  ice.  Great  tongues  of  this  ice  creep,  far 
below  the  snow  line,  down  the  valleys  heading  in  the 
neve,  and  constitute  glaciers. 

Occurrence. — Glaciers  can  only  form  in  regions  of  per- 
petual snow,  and  in  such  regions  large  glaciers  can  form 
only  where  the  snow-fall  is  copious.  Hence,  near  the 
equator  glaciers  are  formed  only  on  mountains  exceeding 
16,000  feet  in  height,  but  they  occur  at  successively  lower 
elevations  in  higher  latitudes,  and  in  the  frigid  zones  on 
hills  of  very  moderate  elevation.  In  any  latitude  glaciers 
are  generally  largest  on  those  eminences  of  sufficient 
height  which  are  first  encountered  by  the  vapor -bearing 
winds  from  the  sea,  and  on  the  sides  of  these  eminences 
which  are  turned  away  from  the  sun ;  that  is,  on  the  north 

(231) 


(«3»i 


GLACIERS    AND    LAKES.        •  233 

side  in  the  northern  hemisphere,  and  on  the  south  side  in 
the  southern  hemisphere. 

The  Himalaya  Mountains,  though  near  the  tropic  of  Cancer,  are 
so  lofty  and  so  well  supplied  with  vapor  by  the  south-west  monsoon 
that  they  bear  immense  glaciers  ;  one  has  a  length  of  over  35  miles. 
The  moderately  high  mountains  of  Alaska,  and  the  low  mountains 
of  Norway,  being  near  the  Arctic  Circle  and  well  supplied  with 
moisture,  also  bear  large  glaciers.  The  Alaskan  glaciers  are  proba- 
bly larger  than  any  others  in  torrid  or  temperate  zones.  On  all  the 
high  peaks  of  the  Sierra  Nevada  and  the  Cascade  Mountains  from 
central  California  northward,  glaciers  are  found ;  on  mounts  Lyell  and 
Dana,  Cal.,  they  are  less  than  a  mile  long ;  Mount  Shasta,  Cal.,  has 
one  two  miles  long,  while  one  on  Mount  Tacoma,  Washington, 
is  ten  miles  in  length,  and  surpasses  in  size  and  grandeur  many 
of  the  Swiss  glaciers.  The  glaciers  of  the  Alps  have  been  visited 
more  than  any  others.  They  are  found  principally  about  Mount 
Blanc,  in  France,  Monte  Rosa,  Finsteraarhorn,  and  the  Bernina 
Alps  in  Switzerland,  and  the  Oetzthaler  Alps  in  the  Tyrol.  Each 
group  has  glaciers  more  than  six  miles  long  and  a  mile  or  two  wide, 
while  Aletsch  Glacier,  on  the  slope  of  Finsteraarhorn,  has  a  length 
of  14  miles.  The  thickness  of  these  glaciers  is  estimated  at  be- 
tween 500  and  1,000  feet.  All  these  glaciers,  however,  sink  into  in- 
significance when  compared  with  those  of  the  polar  regions.  These 
form  at  comparatively  low  elevations,  and,  covering  the  entire  coun- 
try with  a  thick  ice  sheet,  descend  into  the  sea,  where  great  masses 
break  off  and  float  away  as  icebergs.  The  Humboldt  Glacier,  of 
Greenland,  is  thought  to  be  half  a  mile  thick  at  its  sea  front. 

Movement. — Glaciers  creep  downward  at  a  rate  vary- 
ing with  the  slope,  the  season,  and  the  rain-fall,  but  sel- 
dom, if  ever,  at  a  rate  rapid  enough  to  be  perceptible 
without  measurements.  Careful  observation  has  proved 
that  the  movement  of  a  glacier  resembles  in  many  respects 
that  of  the  current  of  a  river.  It  is  faster  on  steep  than 
on  flat  parts  of  its  bed ;  at  the  surface  than  toward  the 
bottom ;  and  near  the  center  than  at  the  sides  of  the  sur- 
face. In  the  curves  of  its  couise,  the  glacier  moves  fastest 
not  at  the  exact  center,  but  at  a  point  in  its  surface  nearer 
the  convex  side  of  the  curve. 


234 


PHYSICAL   GEOGRAPHY. 


The  long  undulating  arrow  follows  the  line  of  most  rapid  motion 
of  "Mer  de  Glace"  in  the  Alps.  The  amount  of  mouement  of  the 
surface  of  the  glacier  -  in  inches,  per  24  hours  in  summer-  is  also 
indicated. 

Fig.  97- 


Along  the  line  on  the  surface  of  the  Mer  de  Glace  where  move- 
ment is  fastest,  the  mean  speed  is  27  inches  a  day  in  summer  and 
about  one  half  as  much  in  winter,  or  about  600  feet  a  year.  Hence, 
this  glacier  requires  more  than  25  years  to  traverse  the  three  miles 
of  its  length.  The  thicker  Greenland  glaciers  move  more  than  30 
feet  a  day,  and  almost  as  fast  in  winter  as  in  summer. 

If  a  square  block  of  ice  be  placed  in  a  mold  of  any  other  shape 
and  subjected  to  hydraulic  pressure,  the  ice  is  crushed  to  powder, 
which  takes  the  shape  of  the  mold,  and  immediately  re-freezes  into 
a  solid  mass  again.  The  phenomenon  is  called  regelation.  The 
amount  of  pressure  required  to  crush  the  ice  is  comparatively  slight, 
but  increases  as  the  temperature  of  the  ice  falls  below  its  melting 
point.  This  experiment  illustrates  why  the  solid  ice  of  a  glacier, 
which  is  brittle  rather  than  plastic,  constantly  moves  downward 
and  conforms  to  the  bends  and  irregularities  of  its  bed  as  if  it 
were  a  truly  plastic  substance  like  wax,  thick  honey,  or  thick  tar. 
The  deepening  snow  of  the  neve  presses  its  lower  layers  into 
solid  ice,  and  at  last  crushes  this  ice  and  squeezes  it  outward 
down  the  glacial  valleys.  But  simultaneously  with  their  move- 
ment, regelation  unites  the  particles  of  crushed  ice  into  a  solid  mass 
again,  which  thus  transmits  pressure  to  the  lower  portions  of  the 
glacier.  The  faster  movement  of  glaciers  in  summer  is  owing  to 
the  fact  that  at  that  season  the  ice  is  nearer  its  melting  point,  and 
hence  yields  more  easily  to  pressure  than  in  winter.  In  addition 
to  these  movements,  the  glacier  slides  bodily  forward  to  a  greater 
or  less  extent,  and  rapidly  corrades  its  bed. 

Ablation  of  the  Surface. — The  surface  of  the  glacier 
is  subject   to  constant   lowering  by  evaporation,   and  the 


GLACIERS    AND    LAKES.  235 

entire  ice  mass,  but  especially  the  surface  below  the  snow- 
line, loses  more  by  melting  in  summer  than  it  receives  by 
snow-fall  in  winter.  The  average  lowering,  or  ablation,  of 
the  surface  of  the  Mer  de  Glace  is  probably  six  inches 
a  day  during  summer.  If,  owing  to  a  succession  of  excep- 
tionally mild  winters  or  hot  summers,  the  amount  melted 
exceeds  the  amount  brought  down  by  movement,  the 
lower  end  of  the  glacier  retreats  up  the  valley.  If  the 
conditions  are  reversed,  the  end  of  the  glacier  advances 
down  the  valley.  The  Swiss  glaciers  have  been  advancing 
since  1875. 

Lateral  moraines. — The  sides  of  valleys  through  which 
glaciers  descend,  being  usually  steep  and  in  regions  of 
great  elevation,  are  exposed  to  great  variations  of  temper- 
ature and  rapid  erosion.  Large  quantities  of  sand,  soil, 
and  rock  fragments  thus  find  their  way  to  the  glacier, 
and  are  carried  by  it  down  the  valley.  This  rubbish  is 
specially  abundant  near  the  sides  of  the  glacier,  where  it 
forms  long  mounds  on  either  edge  of  the  ice.  These  are 
called  lateral  moraines.  When  a  second  glacier  joins  the 
first  from  a  tributary  valley,  the  adjacent  lateral  moraines 
unite  and  are  carried  down  the  center  of  the  united 
glacier  as  a  medial  moraine. 

Each  tributary  glacier-bearing  valley  thus  produces  a  medial 
moraine  on  the  main  glacier  below  its  junction.  The  Mer  de  Glace 
has  five  medial  moraines,  one  of  its  tributaries  having  one  and 
another  two  when  they  join  the  main  glacier.  Medial  moraines  re- 
main distinct  and  well  marked  for  some  distance,  but  are  gradually 
distributed  by  the  differential  motion  of  the  glacier  over  its  entire 
surface.  Large  quantities  of  moraine  matter  protect  the  ice  beneath 
from  rapid  melting;  thus,  medial  moraines  frequently  cover  the 
summit  of  a  ridge  of  ice,  while  great  blocks  of  stone  on  the  glacier 
are,  by  the  melting  of  the  surrounding  surface,  left  perched  as 
"rock  tables"  on  pedestals  of  ice  sometimes  8  or  10  feet  high. 

Terminal  moraines. — At  the  end  of  the  glacier,  the 
moraine   rubbish   is   dumped   upon    the   ground.     If  the 


236  PHYSICAL    GEOGRAPHY. 

glacier  is  stationary  or  advancing,  the  rubbish  accumulates 
to  form  a  curved  ridge  called  a  terminal  moraine ;  but  if 
the  glacier  is  retreating,  the  moraine  matter  is  left  as  a 
coating  of  approximately  uniform  depth  but  very  irregular 
surface,  covering  the  ground  exposed  by  the  retreating 
glacier.  Moraine  matter  left  in  this  generally  distributed 
manner  is  usually  called  glacial  drift  to  distinguish  it  from 
the  same  material  accumulated  into  terminal  moraines. 

Glacial  Abrasion  of  Rocks. — The  rocks  carried  down 
on  the  surface  of  glaciers  undergo  no  friction  and  retain 
their  angularity.  But  vast  numbers  tumble  into  the 
crevasses,  which  at  some  places  open  in  the  glacier  to 
great  depths,  owing  to  irregularities  in  the  slope  of  the 
bed  or  to  the  differential  movement  of  the  glacier.  These 
rocks,  with  others  torn  from  the  bed  or  sides,  work  their 
way  to  the  bottom,  where,  pressed  down  by  the  over- 
lying ice,  they  are  rasped  over  the  rocky  bed  by  the  for- 
ward movement  of  the  glacier;  most  powerful  abrasion 
results,  both  of  the  rocks  embedded  in  the  ice  and  of  the 
underlying  bed  rock.  Long,  continuous  scratches,  or 
stria,  are  indented  upon  each  by  the  harder  particles  in 
the  other,  while  the  exceedingly  fine  powder  resulting 
from  the  abrasion  acts  like  emery  powder,  and  gives  the 
rock  over  which  the  glacier  moves  a  smooth  and  polished 
surface.  As  a  result  of  this  abrasion,  the  ordinary  V-shape 
of  valleys  is  often  changed  into  a  U-  shape,  the  rock  of 
their  bottom  and  sides  is  planedvand  worn  down,  and  all 
their  sharp  angles  removed ;  and  where  the  bed  rock  is 
soft,  it  may  be  hollowed  out  into  deep  basins,  while,  where 
relatively  hard,  it  is  worn  into  smooth,  dome-shaped  emi- 
nences striated  in  the  direction  of  ice  movement. 

The  melting  and  lowering  of  the  glacier's  surface  sometimes 
leaves  its  lateral  moraines  stranded  on  the  valley  sides  to  mark  a 
former  height  of  its  surface.     The  rock  tables  on  the  surface,  or  the 


GLACIERS    AND    LAKES.  237 

worn  and  rounded  bowlders  in  the  body  of  a  glacier,  are  also  some- 
times left  stranded  on  the  steep  valley  sides  among  rocks  of  an  en- 
tirely different  kind.  These  are  called  "perched"  or  "erratic" 
rocks,  and  are  sometimes  left  on  such  precarious  foundations  that 
the  slightest  push  would  apparently  be  sufficient  to  set  them  in  mo- 
tion down  the  slope.  When  glacial  drift  is  removed  from  in  front 
of  a  retreating  glacier,  the  bed  rock  is  always  found  to  be  smoothed, 
polished,  and  striated. 

Glacial  Streams. — A  stream  of  water  always  issues 
from  the  lower  end  of  glaciers.  It  is  derived  partly  from 
springs,  partly  from  surface  waters  higher  up  the  glacial 
valley,  but  chiefly  from  the  melting  of  the  ice.  The  water 
is  charged  with  an  extremely  line  light  gray  silt,  formed 
by  the  constant  abrasion  of  the  rocks,  which  gives  it  a 
peculiar  milky  color,  and  it  retains  this  peculiarity  for  a 
long  time.  This  sediment  forms  a  deposit  of  stiff,  bluish 
clay,  quite  impermeable  by  water,  and  in  marked  contrast 
to  the  yellow  mud  deposited  by  rivers  generally. 

Former  Extent  of  Glaciers. — Indications  of  glacial 
action  on  the  valley  sides  high  above  the  present  surface 
of  glaciers,  and  the  occurrence  of  old  terminal  moraines, 
drift,  polished  rock  surfaces,  erratics,  etc.,  not  only  far 
beyond  the  end  of  glaciers,  but  over  vast  regions  hundreds 
and  even  thousands  of  miles  from  any  existing  glacier, 
prove  that  at  a  comparatively  recent  period  in  the  past, 
glaciers  had  a  much  greater  extent  than  at  present.  The 
whole  northern  half  of  North  America  and  Europe  are 
thus  glaciated.  The  mountain  summits  are  striated  and 
polished,  and  the  lowlands  are  deeply  buried  under  accu- 
mulations of  drift.  This  region  in  each  continent  must 
have  been  covered,  as  Greenland  is  to-day,  by  an  immense 
sheet  of  ice,  so  thick  that  only  the  highest  mountain 
peaks  protruded  above  its  surface.  Many  circumstances 
indicate  that  the  movement  of  these  continental  glaciers, 
in  Europe,  was  outward  in  all  directions  from  the  high- 


(238) 


GLACIERS    AND    LAKES. 


2  39 


lands  of   Norway,   while  in  the  United  States  the  move- 
ment was  from  the  Canadian  Height  of  Land. 

The  southern  limit  of  this  vast  glacier  can  not  be  determined 
exactly.  A  terminal  moraine  (Fig.  98),  from  one  to  several  miles 
broad,  has  been  traced  west  from  Cape  Cod  through  the  inter* 
vening  states  into  the  Dakotas  and  the  Dominion  of  Canada,  and 
forms  a  limit  which  the  ice  certainly  reached.  But  glacial  drift  ex- 
tends many  miles  south  of  this  moraine  in  some  localities. 


Fig  98. 


Effects  on  the  Relief  of  the  Land. — The  thickness 

of  this  glacial  drift  has  been  ascertained  in  many  localities 

in  the  United  States,  and  has  been  found  to  vary  from  a 

few  feet  to  four  or  five  hundred  feet.       It   is  generally 

p.  G.-14. 


240 


PHYSICAL    GEOGRAPHY. 


thickest  in  the  vicinity  of  the  moraine,  and  is  generally 
thicker  in  the  valleys  than  on  the  higher  land.  Almost 
all  the  valleys  in  the  drift  regions  are  more  or  less  filled 
with  drift  gravel,  sand,  and  the  peculiar  blue  clay  of 
glacial  origin.  The  drift  very  generally  fills  not  only  the 
bottom  of  valleys,  and  forms  the  bed  of  the  stream,  but 
frequently  forms  a  series  of  terraces  along  either  side  of 
the  valley  to  a  height  of  several  hundred  feet  above  the 
stream,  which  indicate  the  amount  of  drift  removed  by 
the  stream  since  the  glacial  period  (Fig.  99). 


Fig.  99. 

Cincinnati  is  built  upon  two  such  drift  terraces  of  the  Ohio  valley, 
at  elevations  of  65  and  130  feet  above  low  water  in  the  river.  The 
bed  of  the  Mississippi  River  at  La  Crosse  and  Prairie  du  Chien  is 
more  than  100  feet  above  bed  rock,  and  of  the  Rock  River  at  Janes- 
ville,  Wis.,  more  than  250  feet. 

The  drift  deposit,  especially  where  greatest,  in  the  region  from  the 
moraine  northward,  entirely  blocked  up  and  buried  many  old  val- 
leys, destroying  the  ancient  drainage  lines,  and,  by  its  own  irregu- 
larities, presented  a  new  and  peculiar  surface,  formed  in  disregard 
of  drainage  demands.  In  these  irregularities  water  collected,  giving 
rise  to  innumerable  lakes  (Fig.  98),  which  are  the  special  feature  of 
the  region  north  of  the  moraine  in  both  America  and  Europe. 

Formation  of  the  Great  Lakes. — While  thousands 
of  small  lakes  in  and  north  of  the  moraine  region  occupy 
simple  depressions  in  the  surface  of  the  drift,  the  larger 
lakes,  including  the  American  Great  Lakes,  probably  oc- 
cupy old  preglacial  valleys  of  atmospheric  erosion,  which, 
however,  were  modified  by  movements  of  the  earth's 
crust  and  greatly  broadened  and  deepened  by  the  abra- 


GLACIERS  AND   LAKES.  24 1 

sion  of  the  glaciers  themselves,  the  bulk  of  the  drift  south 
of  these  lakes  being  the  material  so  removed,  *j| 

There  are  indications  which  render  it  not  unlikely  that  the  pre- 
glacial  valleys  of  lakes  Michigan  and  Superior  were  tributary  to  the  , 
Mississippi;  the  Michigan  valley  possibly  south-westward  through 
Illinois,  where  an  ancient  valley,  completely  obliterated  by  a  depth 
of  200  feet  of  drift,  has  been  traced ;  and  the  Superior  valley,  either 
westward  in  the  vicinity  of  St.  Croix  River,  where  the  drift  is  very 
thick,  or  southward  by  some  deep,  narrow,  and  as  yet  undiscovered 
valley,  across  the  upper  peninsula  of  Michigan  into  the  Michigan 
valley.  The  Huron-Erie-Ontario  valley  probably  found  an  outlet 
through  the  St.  Lawrence.  There  are  strong  indications  of  the  ex- 
istence of  a  deeply  buried  and  concealed  valley  connecting  these 
lakes.  The  filling  of  these  natural  channels  at  places  divided  the 
valley  into  separate  basins,  and  forced  the  waters  in  each  basin  to 
seek  a  new  channel  at  the  lowest  point  of  its  water-shed. 

Lakes. — Whenever  the  water  of  a  stream  system,  in 
its  downward  course  over  the  land,  meets  an  obstruction 
to  its  further  advance,  its  current  is  checked,  and  it  tends 
to  accumulate  on  the  upper  side  of  the  obstruction  to  form 
a  pond  or  lake.  The  constant  addition  of  water  from  the 
stream  tends  to  raise  the  level  surface  of  the  lake  to  the 
lowest  point  at  which  the  water  can  escape  through  or 
over  the  obstruction  to  form  an  outlet.  As  the  surface  of 
the  land  is  generally  sloping  and  seldom  precipitous,  a  very 
slight  rise  of  the  lake  usually  occasions  a  great  increase 
both  of  its  width  and  length,  and  hence  of  the  water  sur- 
face exposed  to  evaporation.  It  thus  sometimes  happens 
that  before  the  lake  rises  to  a  point  at  which  it  can  find 
an  outlet,  the  increased  evaporation  from  its  surface  equals 
the  amount  of  water  constantly  added  by  tributaries. 
In  this  case  the  water  surface  can  rise  no  higher,  and  a 
lake  lying  in  an  inland  basin — that  is,  a  lake  having  no 
outlet — is  formed.  Thus,  lakes  may  be  divided  into  two 
classes:  (1)  those  having  outlets,  and  (2)  those  having  no 
outlets.     It  is  an  almost  invariable  rule  that  lakes  with 


242  PHYSICAL    GEOGRAPHY. 

outlets  contain  fresh  water,  while  lakes  without  outlets 
contain  salty  or  bitter  and  undrinkable  water. 

Fresh  Water  Lakes. — Since  the  water  of  all  streams 
contains  more  or  less  mineral  matter  in  solution,  and  since, 
upon  evaporating,  water  leaves  all  impurities  behind,  the 
greater  relative  evaporation  from  the  wider  lake  surface 
tends  to  increase  the  proportion  of  dissolved  impurities  in 
the  lake  water;  hence,  lake  water  usually  contains  more 
matter  in  solution  than  the  average  water  of  its  tributary 
streams.  When  the  lake  has  an  outlet,  however,  this 
difference  is  so  slight  that  the  taste  of  the  lake  water  is 
not  usually  affected,  and  the  difference  does  not  increase 
beyond  a  certain  point,  for  the  impure  lake  water  is  con- 
stantly escaping  by  the  outlet,  while  purer  water  is  con- 
stantly entering  the  lake  through  its  tributaries. 

Salt  Water  Lakes. — In  lakes  having  no  outlets,  the 
constant  loss  of  pure  water  by  evaporation,  and  the  con- 
stant addition  of  the  small  proportion  of  mineral  matter 
dissolved  in  the  tributaries,  causes  a  constant  increase  of 
the  mineral  matter  in  the  lake  water,  until,  eventually,  it 
becomes  saturated  with  some  mineral,  that  is,  can  hold  no 
more  of  this  mineral,  though  other  minerals  present  may 
continue  accumulating.  Further  accessions  of  the  satu- 
rating mineral  are  deposited  in  solid  crystals  on  the  lake 
bottom.  Long  before  the  water  becomes  saturated,  the 
mineral  is  in  sufficient  quantity  to  have  imparted  to  the 
water  its  peculiar  taste,  if  it  has  any.  The  proportion  of 
dissolved  minerals  in  various  waters  is  here  given: 

i  barrel  of  average  fresh  water  contains  about  j^fo  of  a  qt.  of  minerals. 
I      '•      "       **        ocean  water       "  "  3  quarts     "         " 

I      "      "       "        Dead  Sea  water'*         "        22       "        " 

The  kinds  of  mineral  in  any  salt  lake,  and  the  rela- 
tive amount  of  each,  depend  partly,  of  course,  upon  the 
character  of  the  rocks  composing  its  tributary  basin,  and 


GLACIERS    AND    LAKES. 


243 


partly  upon  the  effect  which  different  minerals  in  solution 
have  upon  each  other.  Some  minerals  limit  the  ability  of 
water  to  hold  in  solution  certain  other  minerals,  while 
they  entirely  prevent  the  water  from  holding  in  solution 
still  other  minerals.  The  principal  minerals  in  solution, 
and  their  proportion  (by  weight)  are  given  below. 


Bodies  of  Salt  Water. 


Kara  Booghaz  (gulf  of  Caspian) 
Dead  Sea  (average)     .... 

Great  Salt  Lake 

Mediterranean  Sea  (average)    . 

Open  Ocean   

Black  Sea 

Open  Caspian  Sea 

Average  proportions, 


Total  in 

Solution. 

Lbs.  per  1,000 


285 

243 
I50 

38 

35 
18 

13 


Common 
Salt. 


Chlorides 

and 
Sulphates 
Magnesia 
and  Lime. 


29% 
26 

79 
78 
78 
79 
63 


67/0 
70 
10 
15 

*9 

16 

29 


All 
Other 


4% 
4 
11 

7 
3 
5 
8 


62% 


32% 


6% 


Bodies  of  Fresh  Water. 

Total  in 

Solution. 

Lis.  per  1,000 

Common 
Salt. 

Carbonates 
Lime  and 
Magnesia. 

All 
Other 

Average  River  Water  .... 
Lake  Michigan 

t?tW 

1000 

3% 
4 

64% 
78 

33fo 
18 

Average  proportions. 


3*A%      71%     25^$ 


It  might  be  expected  that  the  proportion  of  the  different  minerals 
in  salt  lake  water  would  be  the  same  as  in  its  fresh  water  tributaries, 
but  simply  increased  in  quantity.  The  above  table  indicates,  how- 
ever, that  94%  of  the  mineral  in  the  salt  waters  consists  of  com- 
mon salt,  and  the  chlorides  and  sulphates  of  magnesia  and  lime, 
while  in  the  fresh  waters  these  minerals  form  less  than  30%,  since 
71%  is  composed  of  the  carbonates  of  lime  and  magnesia.  Not  only 
are  aquatic  plants  and  animals  constantly  robbing  water  of  its  dis- 
solved carbonate  of  lime,  but  when  the  very  slight  quantities  of 
chlorides  and  sulphates  usually  found  in  fresh  waters  have  accumu- 
lated to  a  certain  extent  in  the  lake  or  sea,  they  cause  the  water  to 


244 


PHYSICAL  GEOGRAPHY. 


deposit  almost  all  of  its  carbonates,  thus  leaving  the  salty  and 
bitter  chlorides  and  sulphates  in  excess  in  the  solution.  When 
certain  chlorides  and  sulphates  accumulate  still  more,  they  act  in  a 
similar  manner  upon  each  other,  and  thus  cause  a  deposit  of  much 
of  the  common  salt  (chloride  of  sodium).  Thus,  owing  to  the  large 
proportion  of  chloride  of  magnesia  in  the  water  of  Kara  Booghaz 
and  the  Dead  Sea,  it  can  not  hold  nearly  as  much  salt  as  that  of 
Great  Salt  Lake,  in  which  less  chloride  of  magnesia  has  as  yet  ac* 
cumulated.  This  is  indicated  graphically  in  Fig.  ioo.  This  diagram 
also  indicates  that  the  Dead  Sea,  Great  Salt  Lake,  and  parts  of  the 
Caspian — all  of  them  simply  lakes  without  outlets— contain  a  vastly 


KARA  BOOGHAZ 
DEAD  SEA 
GT.SALT  LAKE 

MtDITIRBANEAN 

OPEN  OCEAN 
BLACK  8EA 

OPEN  CASPIAN     F=^1 
BIVER  WATER       \ 
LAKE  MICHIGAN  0 


Fig.  ioo.— Amount  and  Proportion  of  Dissolved  Minerals  in  Various  Waters, 


greater  proportion  of  mineral  matter  in  solution  than  the  water  of 
the  ocean  or  of  its  arms — the  Mediterranean  and  Black  seas.  As 
the  surface  of  both  the  Caspian  and  Dead  seas  is  considerably 
below  sea-level,  it  is  thought  they  may  at  one  time  have  been  arms 
of  the  ocean,  which  have  been  separated  from  the  ocean  by  the  up- 
heaval of  the  intervening  region.  But  even  if  this  is  the  case,  by 
far  the  greater  part  of  their  intense  "saltness"  is  due  to  the  same 
causes  that  would  gradually  convert  any  fresh  water  lake  into  a  salt 
lake  if  its  outlet  were  permanently  closed. 

The  checking  of  the  current  of  tributaries  upon  en- 
tering a  lake  causes  a  deposit  of  more  or  less  of  the  solid 
particles  held  in  suspension.  Hence,  all  lakes  are  being 
gradually  obliterated,  not  only  by  the  corrasion  of  the  out- 
let channel,  which  tends  to  lower  the  lake  surface,  but  by 
the  deposit  of  sediment,  which  tends  to  fill  up  the  lake 
basin.  On  account  of  this  deposit,  the  water  of  every 
lake  and  of  its  outlet  is  clearer  than  that  of  its  inlet 
streams.     Relatively  large  lakes  usually  contain  very  clear 


GLACIERS    AND    LAKES.  245 

water,  since  they  have  the  slowest  current  and  the  least 
amount  of  matter  in  suspension.  When,  in  addition  to 
the  large  relative  volume  of  the  lake,  the  water  of  the 
tributaries  is  clear,  as  is  usually  the  case  in  regions  of  hard 
rock,  the  water  of  the  lake  is  exceptionally  limpid  and 
transparent. 

Lakes  fed  by  streams  flowing  from  glaciers — as  Geneva,  Mag- 
giore,  Como,  and  many  others — have  usually  a  beautiful  blue  color, 
but  really  their  water  is  less  clear  than  that  of  lakes  of  large  relative 
volume  fed  by  ordinary  streams.  Much  of  the  silt  of  glacial 
streams  is  so  fine  that  the  slight  current  of  the  lake  can  hold  it  in 
suspension.  These  fine  particles  in  suspension,  by  reflecting  only 
the  blue  rays  of  light,  give  the  water  its  peculiar  color,  just  as  the 
fine  particles  of  the  air  give  the  sky  its  azure  tint  (see  page  104). 

Effect  on  Floods.— Floods  in  tributary  streams,  upon 
entering  a  lake,  are  spread  out  over  its  relatively  great  area, 
and  thus  do  not  materially  raise  its  level  surface.  Hence, 
neither  lakes,  nor  streams  issuing  from  them,  are  subject 
to  such  great  variations  of  level  between  low  and  high 
water  as  are  streams  tributary  to  lakes,  or  those  in  whose 
course  no  lakes  occur. 

Thus,  there  are  no  such  great  floods  in  the  Great  Lakes  or  their 
outlet,  the  St.  Lawrence,  as  occur  annually  in  the  Ohio,  Missouri, 
and  other  rivers  whose  courses  contain  few  or  no  large  lakes.  The 
mean  annual  fluctuation  between  high  and  low  water  in  the  Great 
Lakes  is  less  than  1%  feet.  In  the  Ohio,  at  Cincinnati,  it  is  more 
than  50  feet. 

Temperature  of  Lakes. — The  temperature  of  the  sur- 
face water  in  lakes  varies  with  the  seasons,  but  on  account 
of  the  great  specific  heat  of  water  it  does  not  vary  so  rap- 
idly, and  hence  not  to  so  great  an  extent,  as  that  of  the 
overlying  air  or  the  neighboring  land  surface.  During  the 
summer  the  water  is  generally  cooler  than  the  air,  which 
it  therefore  tends  to  cool,  while  during  the  winter  the 
water  tends  to  keep  the  adjacent  air  warm.     If  the  winter 


246  PHYSICAL   GEOGRAPHY. 

temperature  at  the  surface  of  fresh  water  lakes  falls  to  or 
below  that  of  the  maximum  density  of  water  (p.  25),  the 
temperature  at  the  bottom  is  390,  and,  if  the  lakes  are  deep, 
it  remains  constant  throughout  the  year.  From  this  bot- 
tom water  in  such  lakes,  the  temperature  increases  to  that 
of  the  surface  in  summer,  but  may  decrease  to  a  surface 
temperature  of  320  in  winter. 

Distribution, — Lakes  are  much  more  numerous  in 
some  regions  than  in  others.  As  a  general  rule,  lakes  are 
more  numerous  near  water-sheds  than  elsewhere.  Near 
water-sheds,  streams  are  short  and  small;  hence  they 
carry  but  little  sediment,  and  possess  little  power  either  to 
corrade  lake-forming  obstructions  or  to  fill  up  and  obliter- 
ate lake  basins.  There  are  five  kinds  of  regions  where 
lakes  are  particularly  abundant,  the  lakes  being  generally 
fresh  if  the  rain-fall  is  abundant,  but  salt  if  the  rain-fall 
of  the  region  is  scanty. 

(  1  )  Glaciated  regions,  or  those  whose  surface  has  been  cov- 
ered with  irregularities  by  the  abrasion  or  drift -deposit  of  former 
glaciers.  North-eastern  America  and  north-western  Europe  are 
such  regions,  the  old  terminal  moraine  forming  in  the  United  States 
a  sharp  boundary  between  a  vast  lake  region  on  the  north  and  a 
comparatively  lakeless  region  on  the  south    (Fig  98). 

(2)  Mountainous  or  hilly  regions  generally. — In  these  regions 
the  valleys  are  steep  and  narrow.  The  one  quality  favors  the  ero- 
sion of  large  masses  from  the  sides  of  the  valley ;  the  other  permits 
a  comparatively  small  quantity  of  material  to  make  a  high  obstruc- 
tion across  the  valley ;  consequently,  mountain  lakes  are  generally 
very  narrow  and  very  deep.  If  the  five  Great  Lakes  in  the  com- 
paratively flat  portion  of  the  United  States  be  compared  with  the 
five  Alpine  lakes,  Geneva,  Constance,  Como,  Maggiore,  and  Garda, 
it  will  be  found  that  the  Great  Lakes  have  an  average  width  of  one 
fourth,  but  the  Alpine  lakes  of  only  one  ninth  of  their  lengths.  The 
average  of  the  greatest  depths  in  the  Great  Lakes  is  705  feet,  and 
of  the  Alpine  lakes  1,491  feet. 

(3)  Non-glaciated,  basin-shaped  plateau  regions  having  a 
copious  rain-fall.     The  most  remarkable  of  these  extends  south- 


GLACIERS   AND    LAKES.  247 

ward  from   Abyssinia  in  eastern   Africa.     It  contains  a  number  of 
lakes  which  rival  our  Great  Lakes  in  size. 

(4)  Regions  of  scanty  rain-fall  in  general. — These  lakes  are 
seldom  large ;  they  generally  have  no  outlet,  and  hence  contain  salt 
water,  and  many  of  them  are  entirely  evaporated  during  the  drier 
seasons  of  the  year.  Almost  all  the  salt  lakes  of  the  world  occur 
in  regions  having  a  mean  annual  rain-fall  of  less  than  10  inches 
(see  chart,  page  76).  Lakes  are  rather  numerous  in  these  regions 
for  the  same  reason  that  they  are  numerous  near  water-sheds — the 
supply  of  drainage-water  being  small  and  often  intermittent,  the 
streams  make  little  progress  in  cutting  away  lake-forming  obstruc- 
tions or  filling  up  lake  basins.  Among  the  largest  salt  lakes  are 
Caspian,  Aral,  and  Dead  seas  and  Balkash  Lake  of  Asia,  and  Great 
Salt  Lake  of  Utah.  Though  they  lie  in  nearly  rainless  regions, 
they  all  receive  tributaries  from  regions  of  more  copious  rain-fall. 
The  Caspian  Sea  is  five  times  as  large  as  Lake  Superior,  and 
though  it  receives  the  Volga  and  five  smaller  rivers,  the  evaporation 
from  its  vast  area  is  so  great  that  its  surface  lies  85  feet  below  the 
level  of  the  ocean,  and  171  feet  below  the  lowest  point  in  its  water- 
shed. The  main  body  of  the  sea  is  only  brackish,  for  the  great 
shallow  and  nearly  land-locked  gulfs  on  its  eastern  coast — as  the 
Kara  Booghaz,  which  is  nearly  half  as  large  as  Lake  Erie — lose  so 
much  water  by  evaporation  that  a  constant  current  flows  into  them 
and  acts  as  an  outlet  to  the  main  sea.  The  water  of  these  gulfs  is 
much  Salter  than  ocean  water  on  the  same  principle  that  the  water  of 
the  Mediterranean  is  slightly  so.     (See  page  144.) 

(5)  Low  and  sandy  sea-coasts  are  frequently  fringed  with  shal- 
low, brackish  lakes  or  lagoons.  They  occur  along  the  whole  east 
coast  of  the  United  States  south  of  Cape  Cod.  They  are  separated 
from  the  ocean  by  a  narrow  beach  of  sand,  and  receive  the  drainage 
of  the  coast  region  through  small  streams.  The  narrow  beach  is 
the  joint  result  of  the  sediment  of  the  small  streams  and  the  sand 
piled  up  by  the  sea  waves  and  the  wind. 


CHAPTER  XVIII. 

MOUNTAIN    STRUCTURE    AND    LAND   SCULPTURE. 

Every  valley  shall  be  exalted,  and  every  mountain  and  hill  shall  be  made  low : 
and  the  crooked  shall  be  made  straight,  and  the  rough  places  plain  :  and  the  glory 
of  the  Lord  shall  be  revealed. — Isaiah  xl  :  4. 

Mountain  Formation  and  Sculpture. — The  repeated 
uplifts  and  subsidences  of  the  earth's  crust  which  have 
resulted  in  the  gradual  formation  of  the  continental 
plateau,  have  in  general  thrown  the  rock  strata  which 
compose  the  land,  into  a  series  of  wave-like  undulations. 
In  some  extensive  regions  the  undulations  are  so  broad 
and  low  that  the  curvature  is  quite  imperceptible,  the 
strata  lying  apparently  horizontal  or  having  a  very  gentle 
and  uniform  slope  over  great  areas.  This,  in  general, 
is  the  position  of  the  strata  composing  plains  and  plateaus. 
In  the  long  and  comparatively  narrow  mountain  regions, 
however,  which  traverse  each  of  the  grand  divisions,  the 
undulations  are  much  narrower  and  higher.  In  some 
regions  the  strata  have  been  thrown  into  a  succession  of 
huge  open  waves,  while  in  others  the  waves  have  been 
crowded  together  into  a  series  of  closely  compressed  folds, 
so  that  the  strata  stand  directly  on  end  or  are  even  over- 
turned, older  rocks  lying  on  top  of  newer  ones.  Long 
faults  or  fractures  where  the  strata  have  slid  up  or  down 
or  sideways  hundreds  and  even  thousands  of  feet,  are  very 
numerous  in  mountain  regions.  The  rocks  are  generally 
more  or  less  completely  metamorphosed,  hard  gneiss  and 
massive  granite  often  occupying  large  areas,  while  dikes 


MOUNTAIN    STRUCTURE.  249 

and  hardened  outflows  of  lava  are  almost  invariably  found 
in  some  parts  of  the  region. 

The  atmospheric  agents,  and  streams  of  running 
water,  which  are  constantly  disintegrating,  removing,  and 
hence  lowering  all  parts  of  the  land  surface,  are  especially 
energetic  and  rapid  in  their  action  in  these  regions  of  high 
elevation,  and  steep  and  broken  strata.  A  covering  of 
rock,  probably  many  thousand  feet  thick,  has  been  thus 
removed  from  all  parts  of  every  mountain  region.  It  is 
believed  that  a  thickness  of  five  miles  has  been  so  removed 
from  much  of  the  Appalachian  chain,  and  that  at  least 
one  mile  has  been  eroded  from  the  entire  region  between 
the  Rocky  and  Wasatch  mountains. 

This  enormous  erosion  has  seldom  been  uniform, 
however,  over  a  mountain  region.  Other  things  being 
equal,  it  has  been  greatest  where  the  elevations  were 
highest,  the  slopes  steepest,  and  the  rocks  softest.  The 
elevated  tops  and  steep  sides  of  the  folds  of  the  strata 
have  thus  generally  been  most  deeply  eroded,  and  hence 
older  rocks  are  generally  exposed  along  the  crests  than 
along  the  troughs  of  the  folds.  On  account  of  this  great 
but  unequal  erosion,  the  crests  of  the  mountains  do  not 
always  conform  to  the  crests  of  the  folds ;  but,  in  general, 
mountain  ranges  are  simply  the  projecting  remnants  of 
those  portions  of  folds,  which,  on  account  of  the  greater 
hardness  or  the  more  stable  position  of  the  strata,  have 
been  best  able  to  resist  erosion.  The  great  folds  and 
faults  in  the  earth's  crust  have  therefore  determined  the 
direction  and  general  position  of  mountain  ranges,  but 
the  shape  of  a  range — every  peak,  ridge,  spur,  valley,  .and 
canon — is  directly  and  entirely  due  to  erosion. 

Mountains  of  simply  folded  strata. — The  simplest 
mountain  chain  is  that  which  is  carved  from  a  single 
broad  fold  of  very  thick  strata;  such  is  the  Uinta  range 


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MOUNTAIN   STRUCTURE. 


251 


In  the  background  the  Uinta 
/old  is  supposed  to  have  re 
mained  uneroded,  while 
the  foreground  shows 
the    Uinta     Mount- 
ains as  they  exist. 


OF    ROCKS     J""-« 
TERTIARY  (MOST  RECENT} 
MESOZOIC       J 
^CARBONIFEROUS 
IOEVONI»N 
=  OLDEST   (METAMORPHICJ 


by  erosion. 

NORTH 


The  dotted  line 
shows  the  part 
of  fold  removed 


Fig.  101. 


of  Utah  (Fig.  101).  Although  a  thickness  of  3*4  miles 
of  rock  has  been  eroded  from  this  range,  the  deeply  buried 
rocks  now  exposed  along  its  crest  have  been  but  little 
changed  by  metamorphism.  The  three  main  ranges  of 
the  Rocky  Mountains  in  Colorado  have  been  carved  from 
a  series  of  three  broad,  flat,  Uinta-like  folds;  but  in  this 
instance  the  underlying  metamorphic  granite  has  been 
exposed  by  the  erosion,  and  forms  the  mountain  crests, 
while  the  upturned  remnants  of  the  unchanged  stratified 
rocks  which  once  covered  the  higher  parts  of  the  range 
form  a  series  of  "hog  backs"  or  foot-hill  ridges  along  its 
base  (see  Rocky  Mountain  section  opposite). 

Mountains  of  closely  folded  strata.  —  More  fre- 
quently mountain  chains  have  been  carved  from  an  up- 
heaval consisting  of  a  greater  number  of  more  pronounced 
folds.  When  the  structureless  granite  has  not  been  ex- 
posed  by   erosion,    the   projecting    edges    of  the   harder 


252  PHYSICAL   GEOGRAPHY. 

strata  composing  the  folds  form  a  well  defined  and  regular 
series  of  long,  parallel  ridges  of  nearly  uniform  height, 
such  as  constitute  the  Appalachian  chain  of  the  United 
States  (see  page  250),  and  the  Jura  Mountains  of  France 
and  Switzerland.  In  many  of  the  larger  chains  of  the 
world,  however,  the  great  thickness  of  stratified  rock  re- 
moved from  the  top  of  the  folds  has  exposed  either  the 
granite  or  equally  hard  gneiss,  1.  e.,  granite  in  which  some 
of  the  lines  of  stratification  are  still  obscurely  visible.  The 
section  across  the  Alps  (page  250)  may  therefore  be  re- 
garded as  typical  of  all  great  mountain  chains. 


.  /  SNOW  MASS  PEAK 

^V.  \     INVERTED  SS 

OLDER-...  NFWPR  \OLDERjK>CKSJ^/.'. 

ftrdiitjtq 

IN   ELK   MOUNTAINS,    COLORADO.  IN   THE  ALPS,    SWITZERLAND. 

Fig.  10a. 

The  greater  folds  of  most  mountain  regions  are  corrugated  by 
many  minor  plications.  The  portions  of  these  minor  foldings  left 
by  erosion  often  render  the  structure  of  certain  regions  exceedingly 
complicated,  as  indicated  in  Fig.  102.  In  other  localities  the  struct- 
ure is  entirely  concealed  by  great  outflows  of  lava  thousands  of 
feet  thick  and  hundreds  of  square  miles  in  extent ;  such  is  notably 
the  case  in  the  Cascade  Range  in  the  states  of  Oregon  and  Wash- 
ington. 

Mountains  Produced  by  Faultings  of  the  Strata. — 
The  numerous  and  nearly  parallel  ranges  of  the  Great 
Basin,  western  Arizona,  and  northern  Mexico  are  some- 
what different  in  structure.  The  region  between  the  Sierra 
Nevada  and  the  Wasatch  Mountains,  and  extending  from 
Idaho  to  Mexico,  is  composed  of  very  gently  folded  rocks 
deeply  buried  in  places  by  extensive  outflows  of  lava.  A 
series  of  nearly  parallel  fractures,  hundreds  of  miles  long 
and   fifteen   to   thirty    miles   apart,    traverses   this   entire 


AGE   OF    MOUNTAINS.  253 

region  and  divides  it  into  long,  narrow  blocks.  Many 
facts  prove  that  the  whole  region  was  once  more  elevated 
than  at  present,  but  has  subsided  thousands  of  feet,  and 
during  the  subsidence  the  blocks  have  been  tilted  side- 
ways. The  uptilted  side  of  these  blocks,  carved  by  sub- 
sequent erosion,  forms  the  isolated  mountain  ranges  of  the 
region  (see  section  of  Basin  Ranges,   page  250). 

The  rate  of  mountain  upheaval,  either  by  folding  or 
faulting,  is  always  an  exceedingly  slow  process,  the  rocks 
giving  way  a  few  inches,  or  at  most  a  few  feet  at  a  time, 
and  at  very  long  intervals,  as  the  stresses  accumulate.  It 
is  certain  that  such  movements  are  taking  place  at  present 
in  many  mountain  regions,  notably  throughout  the  great 
and  nearly  continuous  highland  on  the  convex  side  of  the 
continental  plateau.  Hence,  notwithstanding  the  enor- 
mous thickness  of  strata  eroded  from  their  tops,  many 
mountains  may  never  have  been  higher  than  they  are  at 
present,  erosion  having  planed  down  the  surface  about  as 
fast  as  it  was  upheaved.  Many  of  the  older  mountains, 
however,  as  the  Appalachians  and  the  mountains  of 
northern  Europe,  in  which  the  upheaval  has  probably  long 
since  ceased,  have  probably  been  greatly  lowered  by  the 
subsequent  erosion. 

The  extreme  slowness  of  upheaval  is  conclusively  proved  in 
many  cases  by  river  gorges,  canons,  or  water  gaps,  cut  directly 
across  the  mountain  range  (page  226).  This  is  plainly  seen  in  the 
Uinta  Mountains.  The  great  Uinta  fold  rose  directly  across  the 
upper  course  of  the  Colorado  River,  but  it  rose  no  faster  than  the 
river  deepened  its  channel ;  hence,  the  river  has  not  been  deflected 
from  its  course,  but  flows  through  the  mountain  in  a  deep  canon, 
which  the  river  cut  as  the  fold  rose. 

Age  of  Mountains. — Most  mountain  chains  have  been 
upraised  by  a  succession  of  these  gradual  uplifts,  separated 
by  long  ages  of  rest  or  subsidence.  The  time  of  the  up- 
heaval which  left  the  region  permanently  above  the  sea 


254  PHYSICAL   GEOGRAPHY. 

determines  the  "age"  of  the  mountain.  The  Appa- 
lachian chain  was  permanently  raised  above  the  sea  before 
the  close  of  the  paleozoic  era;  the  Sierra  Nevada  at  the 
close  of  the  Jurassic  period ;  the  Rocky  Mountains  at  the 
close  of  the  cretaceous ;  and  the  Coast  Ranges  as  re- 
cently as  the  close  of  the  miocene.  Hence,  the  Appa- 
lachians have  been  subjected  to  much  longer  erosion  than 
the  mountains  of  the  west.  The  difference  in  the  length 
of  time  that  various  ranges  have  been  exposed  to  erosion 
may  partly  account  for  the  fact  that  the  oldest  mountain 
chains  are  never  very  high,  while  the  highest  ranges  are 
invariably  among   the   youngest.     The   high   and   nearly 


6j— -         ir^S^^l***^^  ^fl,JllHlll||I|lp^^^^ir^^^^^^^^g>6 

t;sxss^^—^-—~J^'z    2  '  Archaean   and    Granite    j  2  3  4     ~      _ 

Age  of  rocks.  1  —  Archaean,  2 -Silurian,  8  -  Carboniferous,  4  —  Triassic,  5  —  Jurassic, 

6  —  Cretaceous,  7  —  Miocene-  {most  recent)" 
Dotted  lines  indicate  shape  of  eroded  portion  of  fold.     Vert,  scale  6  times  horizontal. 

Fig.  103. 

continuous  ranges  on  the  convex  margin  of  the  conti- 
nental plateau,  and  the  Alps  and  the  mountains  of  Vene- 
zuela are  all  of  recent  upheaval,  while  most  of  the 
detached  and  lower  ranges  near  the  concave  or  Atlantic 
margin  are  much  older. 

The  age  of  mountains  is  determined  by  the  ages  of  the  uncon- 
formable strata  involved  in  the  folds.  A  section  across  the  Black 
Hills  of  S.  Dakota  illustrates  the  process  (Fig.  103).  There  are  two 
lines  of  unconformity  in  this  range  :  between  the  ancient  archaean  (1) 
and  the  overlying  Silurian  (2)  rocks,  and  between  the  comparatively 
recent  cretaceous  (6)  and  the  overlying  miocene  (7).  This  indicates 
the  history  of  the  range,  which  is  typical  of  mountain  ranges  in 
general.  During  archaean  time  the  rocks  of  the  nucleus  were  deposited 
beneath  the  sea  as  horizontal  strata,  were  then  folded  and  metamor' 


AGE    OF    MOUNTAINS.  255 

phosed,  and  the  strata  separated  by  great  protrusions  of  granite. 
They  were  elevated  above  the  sea,  and  the  tops  of  the  folds  removed 
by  erosion.  At  the  close  of  archsean  time  began  z.  long  continued 
subsidence  below  the  sea  again,  during  which  the  successively  more 
recent  rocks  of  the  Silurian,  carboniferous,  triassic,  Jurassic,  and  cre- 
taceous periods  were  deposited  in  horizontal  strata  above  each  other 
on  the  upturned  edges  of  the  archaean  strata.  Then  another  period 
of  upheaval  ensued,  which  gradually  bent  all  the  horizontal  strata 
up  into  the  great  flat  arch  shown  by  the  dotted  lines,  and  raised  them 
permanently  above  the  sea.  This  upheaval  took  place  soon  after 
cretaceous  time,  for  the  unconformable  strata  (7)  contain  fresh  water 
fossils  of  middle  tertiary  age,  and  are  composed  of  material  eroded 
from  the  higher  parts  of  the  arch,  and  must  have  been  deposited  in 
an  old  fresh  water  lake  about  its  base.  Hence,  these  mountains 
are  of  eocene  age,  since  the  uplift  from  which  erosion  is  still  carv- 
ing them  took  place  during  the  first  period  of  the  tertiary  era. 

Thickness  of  Sediments. — The  ragged  and  upturned 
edges  of  strata  in  all  mountain  regions  prove  that  before 
upheaval  and  erosion  the  thickness  of  stratified  rock  in 
these  regions  was  exceptionally  great.  In  the  plicated 
Appalachian  region  the  stratified  rocks  were  eight  miles 
thick,  while  in  Indiana,  where  the  same  strata  are  nearly 
horizontal,  they  are  known  to  be  less  than  one  mile  thick. 
Now,  the  region  where  sediment  is  to-day  accumulating 
fastest  is  a  comparatively  narrow  belt  of  sea  bottom  along 
the  margin  of  the  continental  plateau.  The  incessant 
erosion  of  material  from  the  land,  and  its  constant  deposit 
in  this  "littoral"  belt,  disturbs  the  subterranean  equality 
of  pressures  (page  151),  and  causes  some  regions  of  the 
land  to  rise,  while  the  narrow  marginal  region  of  sea  bot- 
tom subsides  as  weight  gradually  accumulates  upon  it. 
Thus,  during  the  lapse  of  ages,  sediment  many  miles  thick 
may  be  deposited  in  these  regions,  the  water  remaining 
comparatively  shallow  all  the  while. 

No  theory  of  mountain  upheaval  yet  advanced  is 
complete  and  satisfactory.     The  long  and  narrow  shape  of 

P.  G.-if 


256  PHYSICAL  GEOGRAPHY. 

mountain  regions,  their  rough  parallelism  with  coast  lines, 
and  the  comparative  proximity  of  all  the  younger  and 
higher  mountains  to  the  sea,  together  with  the  fact  that 
sedimentary  rocks  are  exceptionally  thick  in  these  regions, 
are  all  to  be  easily  explained  by  supposing  that  mountain 
chains  mark  the  general  position  of  former  marginal  belts 
of  sea  bottom,  which,  after  a  longer  or  shorter  period  of 
subsidence,  underwent  great  but  gradual  upheaval  to 
form  an  elevated  border  to  the  previously  existing  land. 
It  is  generally  believed  that  the  great  mountain  systems 
have  been  formed  by  the  successive  uplifts  of  such  mar- 
ginal regions,  and  there  are  indications  that  the  upheaval 
and  accompanying  plication,  folding  and  faulting  of  the 
strata  are  primarily  due  to  the  gradual  increase  of  subter- 
ranean temperature,  and  the  consequent  resistless  expan- 
sion of  the  deeply  buried  rocks  in  such  localities.  But 
satisfactory  reasons  have  not  yet  been  found  to  account  for 
the  vast  and  relatively  local  variations  of  temperature  in 
these  thick  accumulations  of  sediment,  required  to  convert 
them  from  regions  of  subsidence  into  regions  of  excep- 
tionally great  elevation. 

It  is  not  impossible  that  the  accessions  to  the  width  of  the  conti- 
nental plateau,  caused  by  the  upheaval  of  the  marginal  belt  of  sea 
bottom,  have  taken  place  alternately  on  its  two  sides.  The  most 
recent  accessions  have  been  on  the  convex  or  Pacific  side,  which  in 
general  seems  to  be  still  rising,  and  in  consequence  of  its  elevation 
the  streams  and  sediment  of  most  of  the  land  have  been  directed 
into  the  marginal  region  of  the  concave  or  Atlantic  side,  where, 
therefore,  the  foundations  of  the  great  mountain  chains  of  the 
future  are  possibly  now  being  laid.  Indeed,  the  site  of  such  a 
chain  is  possibly  already  marked  out  by  the  Lesser  Antilles  01 
Windward  Islands. 

Land  Sculpture. — While  erosion  has  been  greatest  in 
mountain  regions,  the  whole  surface  of  the  land  has  prob- 
ably been  lowered  many  hundreds,  or  even  thousands,  of 


LAND    SCULPTURE. 


257 


feet  by  the  rains,  frosts,  and  winds  of  countless  centuries ; 
and  the  position  and  relative  hardness  of  the  different 
rock  strata,  by  influencing  their  resistance  to  erosion,  have 
determined  the  alternation  of  hill  and  valley  in  the  low- 
land regions  of  comparatively  horizontal  rock  strata. 

In  nearly  horizontal  strata,  the  corrasion  of  the  main 
valleys  leaves  the  intervening  region  as  broad,  flat-topped 
plateaus.  The  multiplication  and  deepening  of  tributary 
valleys  eventually  cuts  up  the  plateau  into  irregular  series 
of  hills,  with  rounded  outlines  and  nearly  uniform  height. 
The  western  part  of  the  Appalachian  table-land  from  New 
York  to  Alabama  is  thus  cut  up,  while  the  eastern  part 


f«0  OCWfTV-A  80  V  E  -  S  E  A- 


MESA  OF  SAN  MATEO  MTS 


WM  -  Hard  Lava  EE3  =     Much    softer    strata 

Fig.  104. — Lava-capped  Mesas  in  North-western  New  Mexico. 

still  retains  more  of  its  true  plateau  character.  Where 
the  different  strata  vary  greatly  in  hardness,  the  outline 
of  the  hills  is  more  regular,  the  harder  strata  forming 
lines  of  cliff  along  their  tops  and  sides.  In  the  regions 
of  lava  outflows  in  the  West  isolated,  flat-topped  "mesas" 
or  "table  mountains"  are  numerous,  the  hard  lava  resist- 
ing the  erosion  which  lowers  the  surrounding  regions  (Fig. 
104).  Where  the  horizontal  strata  are  very  soft,  as  in  the 
numerous  extensive  regions  of  "bad  lands"  of  the  West, 
erosion  has  carved  the  surface  into  great  numbers  of 
steep,  isolated  cones  and  pinnacles,  whose  soft  sides  are 
scored  by  the  rain  streamlets  into  countless  straight 
grooves.  These  upright  flutings,  together  with  the  lines 
of  horizontal  bedding,  suggest  regularly  laid  masonry,  and 


258 


PHYSICAL  GEOGRAPHY. 


Fig.  105.— Forms  of  Erosion  in  Washakie  Bad  Lands,  Wyoming. 

give  the  isolated   masses,   when  seen  at  a  little  distance, 
the  aspect  of  a  gigantic  city  in  ruins  (Fig.   105). 

Gently  Inclined  Strata. — A  common  effect  of  erosion 
on  gently  inclined  strata  is  shown  in  the  diagram  (Fig. 
106).     A  succession  of  long,    parallel   lines   of   cliff  are 


-  Soft  Strata 
Fig.  106. 


=  Original  Surface 


formed,  separated  by  plains  often  many  miles  in  width, 
which  drain  toward  the  base  of  the  cliffs  above.  The 
surfaces  of  the  plains  are  the  harder  strata  which  form  the 
crest  of  the  terminating  cliff.  The  gradual  breaking  away 
of  fragments  slowly  carries  the  lines  of  cliff  backward  down 
the  incline  of  the  strata.  The  wash  of  rain  torrents 
carves  the  face  of  the  cliffs  into  a  succession  of  deep  bays 


LAND    SCULPTURE. 


259 


with  bold  promontories  between.  The  widening  of  adja- 
cent bays  frequently  detaches  the  ends  of  the  promontories, 
which,  by  the  recession  of  the  main  cliff,  are  thus  left  as 
isolated  "buttes"  far  out  upon  the  plain  below.  Gradual 
weathering  eventually  disintegrates  the  hard  capping 
strata  of  the  buttes,  and  the  butte  rapidly  disappears. 
Such   lines  of  cliff,   sometimes  2,000  feet  high  and  hun 


Fig.  107.— Vermillion  Cliffs,  Utah,  showing  Outlying  Buttes. 


dreds  of  miles  long,  are  very  common  in  the  Colorado 
Plateau  district  (Fig.  107).  When  some  of  the  cliff- 
forming  strata  are  conglomerate  (rock  composed  of  great 
bowlders  cemented  together),  its  disintegration  and  reces- 
sion frequently  leave  detached  high,  slender  "needles" 
of  soft  strata,  capped  by  a  hard  bowlder.  These  needles 
are  frequently  hundreds  of  feet  high,  and  stand  until  the 
gradual  weathering  of  the  soft  strata  diminishes  the  sup- 
port of  the  capping  bowlders,  which  at  last  topple  over 
and  the  rains  rapidly  reduce  the  needles.  Other  pecu- 
liarities in  the  relative  hardness  of  the  various  rocks  cause 
the  outlyers  of  the  receding  cliffs  to  weather  into  great 
natural  arches  and  other  fantastic  forms  (Fig.   108). 

In  sharply  folded  strata,  the  tendency  of  erosion  is 
always  to  wear  away  the  top  of  the  fold  most  rapidly,  for 


260 


PHYSICAL    GEOGRAPHY. 


Fig.  108. — Various  Fantastic  Forms  of  Erosion. 


not  only  is  the  rock  in  that  locality  most  apt  to  be  greatly 
fissured,  cracked,  and  weakened  by  the  strains  in  folding, 
but  the  position  of  the  strata  is  an  unstable  one,  for  when 
erosion  has  excavated  a  valley  in  the  trough  of  a  fold  (Fig. 

109)  the  inclined  strata  on  the 

sides  thus  deprived  of  their 

support  tend  to  move  down- 

g* IOQ'  ward   as   a    land-slide,  which 

partially  or  wholly  fills  the  valley  and  delays  the  erosion 

of  the  trough,   while  erosion  proceeds  with  undiminished 


LAND    SCULPTURE. 


26l 


activity  on  the  crests  and  sides  of  the  folds.  This  more 
rapid  lowering  of  the  surface  under  the  crests  than  under 
the  troughs  of  the  folds  is  very  conspicuous  in  all  mountain 
regions  of  sharply  folded  strata,  and  the  process  has  gone 
so  far  in  the  older  mountains,  such  as  the  Appalachian, 
that  the  present  ranges  usually  occur  either  along  the 
troughs,  or  are  composed  of  specially  hard  strata  on  the 
inclined  sides  of  the  folds,  while  in  younger  mountains, 
such  as  the  Juras,  the  tops  of  the  folds  have  not  yet  been 
so  greatly  lowered ;  but  before  they  have  suffered  erosion 
as  long  as  the  Appalachians,  the  present  mountain  sum- 


THE     OLD    APPALACHIAN     RIDGES    OF     PENNSYLVANIA 


Fig.  no. 

mits  will  probably  be  valleys,  while  the  site  of  some  of 
the  present  valleys  may  be  occupied  by  ridges.  In  old 
mountains,  therefore,  such  as  the  Appalachians  and  the 
mountains  of  northern  Europe,  most  of  the  strata  which 
occupied  an  unstable  position  have  been  removed,  and 
consequently  land-slides  are  of  rare  occurrence.  In 
younger  mountains,  however,  such  as  the  Alps  and  the 
Sierra  Nevada,  many  of  the  strata  still  remain  in  the  un- 
stable position,  and  extensive  land-slides  are  numerous 
and  often  very  disastrous. 

Canoe-shaped  Valleys. —  Though  many  miles  long, 
the  individual  folds  of  mountain  regions  are  seldom  nearly 
as  long  as  the  whole  disturbed  region  in  which  they  occur. 


262 


PHYSICAL   GEOGRAPHY. 


Fig.  in.— Formation  of  Canoe-shaped  Valleys. 


In  reality,  each  fold  is  a  greatly  elongated  dome.  A 
sketch  of  such  a  dome-fold  is  shown  in  Fig.  m  (A), 
its  internal  structure  being  shown  at  B.  The  hard  strata 
left  projecting  by  the  erosion  of  such  a  fold  form  mountain 
ridges  which  gradually  approach  and  unite  at  either  end  of 
the  dome-fold  (C),  thus  inclosing  one  of  the  lozenge  or 
canoe-shaped  valleys  so  common  in  central  Pennsylvania. 
Frequently  several  minor  folds  are  pressed  closely  together, 
and  this  corrugated  surface  carried  up  into  a  great  dome- 
fold  (D).  In  this  case,  the  erosion  of  the  fold  would  leave 
the  projecting  hard  strata  in  the  form  of  a  mountain  ridge 
having  curious  zigzags  in  its  trend    (E). 

Water  Gaps. — The  drainage  of  such  confined  valleys 
usually  escapes  through  the  inclosing  mountain  by  a 
narrow  notch  or  water  gap.  These  water  gaps  have  been 
cut  entirely  by  erosion,  but  frequently  at  a  point  where 
the  hard  strata  forming  the  mountain  rim  have  been  broken 


LAND   SCULPTURE.  263 

and  slightly  displaced  by  a  fault.  Such  a  slight  displace- 
ment has  determined  the  position  of  the  Delaware  Water 
Gap. 

The  granitic  crests  of  high  mountains  generally 
weather  into  a  very  jagged  and  irregular  outline,  —  sharp, 
high  peaks,  alternating  with  relatively  low  passes.  This 
peculiarity  led  to  the  name  sierra — the  Spanish  word  for 
saw.  It  is  largely  due  to  the  absence  in  highly  metamor- 
phic  rocks  of  the  lines  of  stratification,  which,  by  direct- 
ing percolating  water  into  definite  channels,  cause  the 
stratified  rocks  to  disintegrate  with  an  approach  to  reg- 
ularity. 


CHAPTER  XIX. 

EARTHQUAKES. 

Then  the  earth  shook  and  trembled  ;  the  foundations  also  of  the  hills  moved  and 
were  shaken.— Psalm,  xviii  :  7. 

Earthquakes. — The  constant  wearing  away  of  parts  of 
the  earth's  surface  by  erosion,  and  the  building  up  of  the 
other  parts  by  deposit,  causes  a  very  slow  but  incessant 
change  in  subterranean  pressures  and  temperatures  (page 
186).  These  changes  in  pressure,  and  the  tendency  toward 
expansion  or  contraction  accompanying  these  changes  of 
temperature,  place  the  deeply  buried  rocks  every-where  in 
various  states  of  stress  or  strain.  No  region  is  exempt  from 
such  stresses.  For  years  or  centuries  the  stresses  accumu- 
late until  they  become  greater  than  the  rocks  can  bear. 
A  sudden  but  slight  movement  of  the  strata  then  occurs, 
which  relieves  the  stress,  and  for  another  long  period  the 
rocks  remain  practically  stationary,  while  stresses  again 
accumulate.  Thousands  of  such  slight  movements,  dis- 
tributed over  tens  or  hundreds  of  thousands  of  years, 
result  in  great  upheavals  or  subsidences  of  the  earth's 
surface,  with  faultings,  flexures,  or  plications  of  the 
strata.  The  shocks  or  jars  of  each  of  these  slight  but 
sudden  movements  in  deeply  buried  strata  are  rapidly 
transmitted  through  the  rocks  in  all  directions,  and  may 
reach  the  earth's  surface,  where  they  are  felt  over  a  larger 
or  smaller  area  as  an  earthquake. 

The  subterranean  movements  which  cause  great  earthquakes  are 
generally  sufficient  in  amount  to  cause  perceptible  displacement  of 

(264) 


EARTHQUAKES.  265 

the  surface  strata.  Such  displacements  are  generally  of  a  few 
inches  or  a  few  feet  only,  and  usually  consist  of  the  elevation  or 
depression  of  the  strata  on  one  side  of  a  line  of  fault.  Often  the 
movement  causing  an  earthquake  occurs  beneath  the  sea,  and  the 
overlying  water  conceals  the  surface  displacement.  Some  severe 
earthquakes,  and  the  great  majority  of  minor  ones,  are  caused, 
however,  by  subterranean  fractures  or  movements  so  small  that  no 
sensible  alteration  in  the  surface  topography  results,  the  movement 
being  entirely  taken  up  by  the  redistribution  of  internal  stresses. 

Earthquakes  are  very  common ;  it  is  probable  that 
there  is  one  every  hour  of  the  day  in  some  part  of  the 
earth.  They  are  more  frequent  in  some  regions  than  in 
others,  but  there  is  no  region  where  they  may  not  occa- 
sionally be  felt.  In  mountain  regions,  and  especially  in 
the  highest  and  youngest  mountains,  erosion  is  most  rapid, 
and  on  the  sea  bottom,  along  the  margins  of  continents, 
sedimentation  is  greatest;  in  these  regions,  therefore,  sub- 
terranean pressure  and  temperature  changes  are  most  rapid, 
and  earthquakes  are  frequent.  Earthquakes  are  most 
frequent  along  the  convex  or  Pacific  side  of  the  great 
continental. plateau,  which  is  bordered  by  the  highest  and 
youngest  mountains.  Earthquakes  are  least  frequent  in 
comparatively  low  and  level  inland  regions,  as  the  central 
part  of  South  America,  central  United  States  and  British 
America,  Russia,  Siberia,  and  central  Australia,  and  in 
the  sea  bottom  far  from  land.  In  these  regions  respect- 
ively, erosion  and  sedimentation  are  very  slight,  and  occa- 
sion a  relatively  slow  accumulation  of  subterranean  stresses. 

On  an  average,  thirty  or  more  earthquakes  occur  in  the  United 
States  annually.  More  than  500  were  recorded  in  this  country  dur- 
ing the  sixteen  years  between  1872  and  1887,  and  doubtless  many 
others  occurred  which  were  not  recorded,  especially  in  the  sparsely 
settled  region  west  of  the  Rocky  Mountains.  These  earthquakes 
were  distributed  thus: 

West  of  Rocky  Mountains      .     .  240 ;  average,  1  every  24  days. 
East  of  Appalachian  Mountains,  210;         "         "       "     28     " 
Mississippi  Valley 80;        "        ■•      •«     73    " 


266  PHYSICAL    GEOGRAPHY. 

The  regions  shaken  by  the  most  perfectly  recorded  of  these 
earthquakes  are  approximately  indicated  on  the  accompanying  map. 
It  gives  a  graphic  idea  of  how  common  earthquakes  are  even  in  the 
Mississippi  Valley,  which  is  one  of  the  most  stable  regions  on  the 
land  surface  of  the  globe.  Many  earthquakes  west  of  the  Rocky 
Mountains  probably  affected  a  more  extensive  region  than  some  of 
those  indicated,  but  are  not  shown  upon  the  map  because  their 
record  in  that  thinly  settled  region  does  not  indicate  even  approxi- 
mately their  extent. 

Elastic  Waves. — Almost  all  rocks,  and  many  other 
solids,  are  highly  elastic  within  minute  limits ;  that  is,  they 
yield  very  slightly  under  great  stress  or  pressure,  and  re- 
gain their  former  shape  or  volume  immediately  when  sud- 
denly relieved  from  stress.  It  is  this  property  which 
causes  an  ivory  or  a  glass  ball  to  rebound  when  dropped 
upon  a  hard  surface.  In  consequence  of  the  elasticity  of 
rock,  the  sudden  relief  from  stress  afforded  by  the  occa- 
sional movements  of  subterranean  strata  throws  the  adja- 
cent rock  molecules  into  a  state  of  very  slight  vibration. 
While  the  distance  through  which  the  molecules  vibrate 
is  so  slight  as  to  be  invisible,  the  energy  of  the  vibra- 
tion may  be  very  great  —  nearly  as  great  as  that  of  the 
accumulated  stress  which  caused  the  movement.  The 
vibrating  molecules  communicate  a  similar  but  less  ener- 
getic vibration  to  neighboring  molecules,  and  these  to 
molecules  still  more  distant.  In  this  way  a  thrill  or 
tremor,  called  an  elastic  wave,  is  transmitted  through  the 
rock  from  the  locality  of  the  initial  jar  in  all  directions 
with  wonderful  rapidity  but  gradually  decreasing  energy. 
Upon  arriving  at  the  earth's  surface,  the  energy  of 
the  invisible  vibration  of  molecules  in  the  elastic  wave 
causes  the  visible  movements  of  the  surface  soil  or  the 
sensible  shocks  which  constitute  an  earthquake.  Hence, 
there  are  three  features  to  be  considered  in  regard  to  an 
earthquake:  (i)  the  origin  of  the  jar;  (2)  the  transmission 


(267) 


268  PHYSICAL    GEOGRAPHY. 

of  its  energy  to  the  earth's  surface,  and  (3)  the  effects  of 
this  energy  upon  the  surface. 

The  transmission  of  energy  through  a  solid  by  an  elastic  wave 
may  be  made  manifest  by  placing  some  light  object,  as  a  toy 
marble,  in  contact  with  one  end  of  a  long,  heavy  iron  bar,  and 
striking  the  other  end  of  the  bar  with  a  hammer.  The  blow  may 
not  cause  the  slightest  movement  of  the  heavy  bar  as  a  whole,  yet 
the  molecules  with  which  the  hammer  comes  in  contact  are  thrown 
into  invisible  vibration.  The  vibration  is  transmitted  from  molecule 
to  molecule  through  the  bar,  and  may  impart  sufficient  energy  to 
the  light  marble  to  cause  it  to  start  visibly  and  perhaps  violently 
forward.  If,  instead  of  a  marble,  a  cake  of  moist  clay  be  made  to 
adhere  to  the  end  of  the  bar,  the  transmitted  energy  may  be  suffi- 
cient to  detach  the  clay  cake.  These  experiments  illustrate  per- 
fectly the  three  features  of  an  earthquake :  the  hammer  blow 
represents  the  initial  jar ;  the  invisible  molecular  vibration  propa- 
gated through  the  bar  represents  the  invisible  transmission  of 
energy  as  an  elastic  wave  through  the  heavy  crust  of  the  earth  ; 
and  the  visible  movement  of  the  marble  or  clay  represents  the 
effect  of  this  wave  upon  comparatively  light  surface  objects,  such 
as  the  soil,  buildings,  or  weakly  attached  masses  of  cliffs,  etc. 

The  Origin  of  the  Jar. — All  knowledge  respecting  the 
deeply  buried  origin  of  a  jar  must  be  gathered  from  ob- 
servations of  the  effects  of  the  earthquake  at  the  earth's 
surface;  and  many  circumstances  render  it  exceedingly 
difficult  to  draw  proper  inferences  from  these  observations, 
which  are  themselves  difficult  to  make  with  accuracy.  It 
seems  to  be  true,  however,  (1)  that  the  origin  is  seldom 
more  than  1 2  miles  below  the  surface ;  it  may  occur, 
however,  at  any  depth  less  than  this ;  (2)  that  the  size  of 
the  shaken  region  bears  a  certain  relation  to  the  depth  of 
the  origin,  a  small  shaken  region  always  indicating  a  rela- 
tively shallow  origin  ;  (3)  that  the  energy  of  the  jar  is 
approximately  indicated  by  the  size  of  the  shaken  region, 
a  large  shaken  region  indicating  a  great  accumulation  of 
energy  or  stress  in  the  initial  jar;    (4)   that  the  origin  is 


EARTHQUAKES.  269 

seldom  a  point,  but  generally  a  line  or  narrow  district, 
which  may  be  many  miles  in  length ;  and  (5)  that  the 
subterranean  stresses  are  not  relieved  by  a  single  move- 
ment of  the  strata,  but  rather  by  a  quick  succession  of 
movements,  causing  a  series  of  jars,  and  it  is  such  a  series 
that  causes  an  earthquake.  The  series  lasts  from  a  second 
or  two  to  several  minutes.  The  jars  of  a  series  quickly 
increase  to  a  maximum  of  energy,  and  then  more  gradu- 
ally become  less  energetic. 

The  redistribution  of  internal  stresses  following  the  movements 
of  the  strata  which  cause  a  great  earthquake,  generally  results  in 
lesser  movements  of  other  subterranean  strata  in  the  neighborhood, 
and  thus  originates  minor  earthquakes,  which  may  affect  the  same 
region  at  irregular  intervals  for  a  year  or  more  after  the  occurrence 
of  the  great  earthquake. 

The    Transmission    of    Earthquake    Shocks. — An 

elastic  wave  travels  through  rocks  with  wonderful  rapidity; 
still  its  transmission  occupies  a  certain  amount  of  time. 
Hence,  an  earthquake  shakes  places  which  are  near  to  the 
origin  sooner  than  places  successively  more  distant.  The 
part  of  the  earth's  surface  which  is  nearest  to  the  site  of 
the  subterranean  jar  lies  directly  over  it.  Therefore,  in 
any  earthquake  the  district  in  which  the  shocks  occur 
earliest  is  called  its  epicentrum  (on  the  center).  This  is 
located  at  some  approximately  central  part  of  the  shaken 
region.  At  the  boundary  of  the  shaken  region  the  earth- 
quake occurs  some  seconds  or  minutes  after  the  epicen- 
trum is  shaken. 

The  velocity  of  elastic  waves  is  greater  in  compact 
solids  than  in  those  of  looser  texture.  It  is  about  four 
miles  a  second  in  steel,  about  two  miles  a  second  in  com- 
pact granite,  and  but  800  to  1,000  feet  a  second  in  compact 
sand  or  clay.  The  different  strata  near  the  earth's  surface 
vary  greatly  in  compactness,   but  as  the  depth  increases, 


270 


PHYSICAL    GEOGRAPHY. 


the  weight  of  overlying  rocks  compacts  all  the  strata,  but 
especially  the  more  porous  and  compressible,  until,  at  the 
comparatively  slight  depth  of  a  few  thousand  feet,  all  the 
strata  attain  a  great  and  nearly  uniform  degree  of  com- 
pactness. Hence,  the  earth's  crust  may  be  divided  into 
two  layers  or  shells:  (1)  a  comparatively  thin  outer  shell 
of  exceedingly  various  density,  through  which  elastic 
waves  travel  at  greatly  differing  velocities;  and  (2)  a  thick 
inner  shell  of  great  and  approximately  uniform  compact- 
ness, through  which  the  waves  travel  with  a  nearly  uni- 
form velocity,  which  is  thought  to  exceed  three  miles  a 
second. 


Fig.  112. 


The  diagram  (Fig.  112)  represents  a  section  of  the  earth's  crust, 
SF  the  surface,  and  DL  the  division  (say  at  the  depth  of  1  mile) 
between  the  two  shells.  Suppose  the  outer  shell  to  the  left  of  C  is 
more  compact  than  to  the  right  of  C,  and  the  inner  shell  is  still 
more  compact.  Let  O,  at  a  depth  of  5  or  6  miles,  be  the  origin  of 
an  earthquake,  and  the  spaces  between  adjacent  curved  lines  the 
distance  traveled  by  the  elastic  wave  in  one  second  of  time.  As 
the  wave  spreads  and  enlarges,  it  maintains  a  general  spherical  or 
spheroidal  shape  in  the  inner  shell,  but  becomes  flattened  and  other- 
wise deformed  as  it  passes  through  the. outer  shell  of  varying  com- 
pactness. In  nature,  the  shape  of  the  wave  is  much  more  irregular 
than  represented,  especially  in  the  outer  shell ;  and  it  is  on  account 
of  these  great  irregularities  of  shape,  speed,  and  effect  resulting 
from  the  passage  of  the  wave  through  the  superficial  strata,  that  it 
is  so  difficult  to  discover  the  laws  of  earthquakes  from  surface  ob- 
servations.    The  diagram  indicates  that  the  wave  reaches  the  sur- 


EARTHQUAKES.  27 1 

face  first  at  the  epicentrum  E ;  that  it  then  requires  one  second  to 
spread  to  W  and  V;  two  seconds  for  it  to  reach  ^f  and  Y,  while  the 
wave  does  not  shake  6"  and  F  until  four  seconds  after  E  is  shaken. 

Energy. — Not  only  does  the  earthquake  occur  earliest 
at  the  epicentrum,  but  its  energy  is  greatest  in  this  lo- 
cality. The  energy  is  least  at  the  boundary  of  the  shaken 
area.  The  diagram  (Fig.  1 1 2)  renders  this  evident.  The 
energy  which  causes  the  disturbance  in  every  part  of  the 
shaken  region  was  originally  concentrated  at  0.  It  spreads 
from  0  in  the  elastic  wave.  As  the  wave  enlarges,  the 
energy  is  distributed  over  its  increasing  circumference,  and 
the  amount  at  any  one  point  in  the  wave  constantly 
diminishes;  hence,  the  point  E  in  the  small  circle  2,  2, 
receives  more  energy  than  the  points  X  and  Y  in  the 
larger  circle  4,  4,  and  much  more  than  the  points  5  and 
F  in  the  still  larger  circle  6,  6. 

The  distance  to  which  an  elastic  wave  is  propagated 
depends  (1)  on  the  amount  of  energy  at  the  origin,  and 
(2)  upon  the  compactness  and  uniformity  of  the  strata. 
A  jar  occurring  in  the  outer  shell  might,  on  account  of 
its  nearness  to  the  surface,  cause  an  exceedingly  violent 
earthquake  at  and  about  the  epicentrum,  but  on  account 
of  the  rapid  dissipation  of  energy  in  passing  through 
strata  of  loose  texture,  the  earthquake  would  probably 
affect  but  a  small  surface  area.  If  the  same  amount  of 
energy  should  cause  a  jar  at  a  much  greater  depth,  the 
epicentrum,  being  farther  from  the  origin,  would  be  less 
energetically  shaken,  but  the  elastic  waves  would  spread 
faster  and  farther  through  the  deep,  compact  strata,  and 
might  carry  to  considerable  distances  enough  energy  to 
penetrate  the  thin  outer  shell,  and  thus  cause  the  shaking 
of  a  much  more  extensive  surface  region.  It  seems  prob- 
able, however,  that  most  jars  in  the  inner  shell  are  more 
energetic  than  those  which  occur  in  the  outer  shell,  for  it 


272  PHYSICAL    GEOGRAPHY. 

must,  in  general,  require  a  greater  accumulation  of  energy 
to  cause  movement  in  deeply  buried  strata  than  in  those 
pressed  upon  by  a  less  weight  of  overlying  rocks. 

The  subterranean  explosions  and  the  fracturing  and  Assuring  of 
the  strata,  which  frequently  accompany  volcanic  eruptions,  are  often 
sufficiently  energetic  to  cause  violent  earthquakes  in  the  immediate 
vicinity  of  the  volcano ;  but  such  earthquakes  never  affect  a  large 
region  because  the  origin  is  at  a  comparatively  slight  depth. 

Surface  Effects  of  Earthquakes. — The  vibration  of 
rock  molecules  which  constitutes  an  elastic  wave  consists 
chiefly  of  a  minute  forward  movement  in  the  direction  the 
wave  advances,  and  a  minute  backward  movement  toward 
the  origin.  Consequently,  the  earthquake  at  the  epicen- 
trum  consists  of  an  up-and-down  shaking,  while  at  other 


\ 


r/^ 


o 

8.F.  =  Earths  surface  E.=  Epicentrum 

O.  =  Origin  V.  X.4  Y.=  Points  successively  more  distant 

Double  headed  arrows  =  direction  of  vibration  of  rock  moleciJes 

Fig.  113. 

places  in  the  shaken  region  the  movement  becomes  more 
and  more  horizontal  as  the  distance  from  the  epicentrum 
increases.  This  is  made  plain  by  Fig.  113,  which  also 
indicates  that  the  epicentral  area  s-v,  in  which  the  princi- 
pal movement  is  up  and  down,  forms  a  very  small  part 
of  the  whole  shaken  area,  its  size  increasing  with  the  depth 
of  the  origin. 

The  boundary  of  this  epicentral  district  can  sometimes  be  located 
on  the  ground  with  considerable  accuracy  as  inclosing  the  area 
where  the  relative  violence  of  the  earthquake  has  manifestly  been 
greatest.  When  this  can  be  done,  it  affords  the  best  known  method 
for  calculating  the  depth  of  the  origin. 

Within  the  epicentral  district  the  earthquake  tends 
to  throw  surface  objects  upward.  Men  and  heavy  masses 
of  rock  have  been  thrown  into  the  air,  and  large  trees 


EARTHQUAKES.  273 

have  been  uprooted  and  thrown  upward.  Foundations 
of  brick  or  stone  masonry  under  buildings  in  the  epicen- 
tral  district  are  sometimes  actually  crushed  by  the  sud- 
denness of  the  upthrust  when  the  enormous  energy  of  the 
elastic  wave  arrives  at  the  surface  beneath  them,  just  as  a 
sudden  upward  blow  on  a  suspended  mass  of  wax  may 
crush  and  indent  it,  while  if  the  same  amount  of  energy, 
had  been  applied  more  gradually,  it  would  have  simply 
moved  the  whole  mass  of  wax  without  indenting  it. 

Without  the  epicentral  district,  the  principal  im- 
pulse of  the  earthquake  is  more  nearly  horizontal.  Still 
there  is  some  up-and-down  movement,  and  this  may  im- 
part a  slight  but  yet  sensible  motion  to  the  comparatively 
light  surface  strata,  just  as  sensible  motion  was  imparted 
to  the  clay  cake  on  the  end  of  the  iron  bar.  This  slight 
up-and-down  movement  imparted  to  adjacent  parts  of  the 
ground  in  quick  succession,  as  the  earthquake  spreads 
rapidly  outward,  throws  the  surface  into  an  actual  undu- 
lation or  wave,  similar  to  a  water  wave  that  spreads  out- 
ward from  an  agitated  point  in  the  surface  of  a  pond. 

Cracks  and  Fissures  in  the  Soil. — The  passage  of 
the  crest  of  such  an  earth-wave  or  undulation  often  causes 
fissures  many  feet  deep  to  open  in  the  soil,  which  some- 
times remain  open,  but  more  frequently  open  and  close 
alternately  as  the  crest  and  trough  of  successive  undula- 
tions pass  under  them.  The  conduits  of  subterranean 
water  are  generally  disarranged  by  such  fissures,  and  thus 
the  location  of  surface  springs  is  frequently  changed,  tem- 
porarily or  permanently,  by  an  earthquake,  while  the  un- 
derground pressures  of  the  passing  undulations  often  cause 
the  ejection  from  the  fissures  of  water,  sand,  and  mud  to 
a  height  of  many  feet. 

Buildings  are  caused  to  rock  or  sway  back  and  forth 
by  the  passage  of  such  undulations,  the  oscillation  being 
p.  G.-16. 


2  74  PHYSICAL    GEOGRAPHY. 

greater  in  the  upper  part  of  the  building  than  below,  just 
as  the  part  of  a  ship  which  oscillates  through  the  greatest 
distance  when  a  wave  passes  under  the  vessel,  is  the  top 
of  its  masts.  A  very  slight  movement  of  this  kind  is 
sufficient  to  crack  the  walls  of  rigid  buildings,  and  to  oc- 
casion a  swing  at  the  top  of  high  houses  great  enough 
to  cause  the  downfall  of  chimneys  or  even  of  the  walls 
themselves.  Well-made  frame  buildings,  on  account  of 
the  greater  play  which  they  allow  at  the  places  where  the 
various  timbers  are  joined  together,  are  not  so  apt  to  be 
destroyed  by  earthquakes  as  rigid  brick  or  stone  houses. 
It  is  by  this  quiet  oscillation  of  buildings  that  many  ex- 
tensive earthquakes  are  recognized  over  by  far  the  greater 
part  of  the  shaken  area,  most  or  all  of  the  "shocks"  be- 
coming so  slight  in  the  transmission  to  great  distances 
that  they  are  scarcely  perceptible. 

The  surface  violence  of  earthquakes  varies  greatly  in  closely  ad- 
jacent localities,  owing  to  the  differences  in  texture  of  the  superficial 
strata.  The  localities  where  earthquakes  are  apt  to  be  least  violent 
are  those  situated  near  the  center  of  an  extensive  region  underlaid 
to  a  great  depth  with  strata  of  loose  texture,  for  all  but  the  most 
energetic  waves  are  quenched  in  this  loose  material  before  reach- 
ing the  surface ;  but  if  the  depth  of  the  loose  material  is  only 
slight,  the  locality  is  apt  to  be  more  violently  shaken  than  one  on 
compact  rock,  for  the  elastic  wave  may  impart  to  a  thin  and  com- 
paratively light  layer  of  loose  material  at  the  surface,  as  it  did  to 
the  clay  cake  on  the  iron  bar,  a  sensible  motion,  which  is  apt  to  be 
sufficient  to  destroy  the  most  substantial  buildings. 

Deep  sounds  or  rumblings  frequently  accompany  or 
follow  earthquakes,  especially  in  and  about  the  epicentral 
district.  They  are  caused  by  vibrations  imparted  to  the 
air  by  such  of  the  rock  vibrations  as  are  of  the  proper 
length  and  rapidity  to  excite  in  us  the  sensation  of  sound. 
The  air  transmits  these  vibrations  to  the  ear  precisely  as 
it  does  those  of  the  string  of  a  violin,  and  the  air  vibra- 
tions become  sensible  as  sound  in  both  cases. 


(275) 


276  PHYSICAL    GEOGRAPHY. 

Sea  Waves  Caused  by  Earthquakes. — When  the 
epicentrum  of  an  earthquake  occurs  beneath  the  sea,  the 
upward  impulse  of  the  sea  bottom  may  upheave  the  over- 
lying water  and  cause  a  series  of  sea  waves,  which  spread 
in  all  directions  to  great  distances  with  a  velocity  which 
increases  with  the  depth  of  the  water,  but  which,  even  in 
the  deepest  ocean,  is  not  nearly  so  great  as  the  velocity 
of  an  elastic  wave  through  a  compact  solid.  Hence,  if 
the  earthquake  which  causes  the  sea  wave  is  felt  at  all  on 
land,  it  is  felt  some  time  before  the  arrival  of  the  sea 
wave. 

In  the  deep  open  ocean  these  sea  waves  are  so  long  and  so  low 
that  their  passage  beneath  a  vessel  is  generally  imperceptible;  but  in 
entering  shoal  water,  as  land  is  approached,  the  waves  become 
shorter  and  higher,  after  the  manner  of  the  tide  wr.ves,  and  their 
arrival  at  the  shore  is  indicated  by  the  rapid  rise  of  the  water  above 
its  usual  level.  Such  a  rise  of  50  to  100,  or  even  200  feet,  has  been 
known.  The  greatest  waves  are  produced  by  an  earthquake  caus- 
ing a  very  energetic  disturbance  in  an  epicentral  district  located  not 
very  far  from  the  coast,  and  yet  beneath  deep  water.  These  con- 
ditions are  most  likely  to  occur  on  the  steeply  sloping  convex  margin 
of  the  continental  plateau,  and  hence  great  sea  waves  are  more 
frequent  on  the  abrupt  Pacific  coasts  than  on  the  more  gently  slop- 
ing Atlantic  shores,  though  great  waves  inundated  the  steep  coast 
of  Portugal  after  the  great  Lisbon  earthquake. 

Among  the  earthquakes  which  have  occurred  in  the 
United  States,  four  are  specially  prominent  on  account  of 
the  great  area  over  which  they  were  felt. 

In  181 1  an  earthquake  shook  the  entire  territory  between  western 
Texas  and  Washington  City,  and  the  Gulf  of  Mexico  and  the  Great 
Lakes,  an  area  of  more  than  a  million  square  miles.  It  was  caused 
by  subterranean  movements  which  occasioned  the  settling  to  a 
depth  of  15  or  20  feet  of  a  large  district  about  New  Madrid,  Mo., 
below  the  junction  of  the  Ohio  and  Mississippi  rivers.  Portions  of 
the  sunken  district,  20  miles  or  more  in  length,  were  afterward 
flooded  by  the  river,  and  became  Reelfoot  Lake  in  north-western 
Tennessee,  and  Big  Lake  between  Missouri  and  Arkansas. 


EARTHQUAKES.  2JJ 

In  1872  an  earthquake  was  felt  over  the  Pacific  slope  from  Oregon 
far  into  Mexico,  and  from  the  coast  eastward  to  Utah  and  New 
Mexico.  The  surface  indication  of  the  subterranean  movements 
which  caused  this  earthquake  was  the  tilting  and  shifting  of  a  great 
block  of  the  earth's  crust  40  miles  long  and  one  fourth  of  a  mile 
wide  in  Owens  Valley,  Cal.,  at  the  east  base  of  the  Sierra  Nevada. 
This  block  settled  about  25  feet  along  its  western  side,  and  about  5 
feet  along  its  eastern  side.  Many  houses  in  the  town  of  Inyo,  near 
the  epicentral  district,  were  destroyed,  and  several  lives  were  lost. 

In  1886  an  earthquake  occurred  which  shook  the  region  from 
Wisconsin  to  Cuba  and  the  Bermuda  Islands,  and  from  Maine  to 
the  mouth  of  the  Mississippi,  an  area  of  nearly  3,000,000  square 
miles.  Its  epicentrum  was  about  1 5  miles  north-west  of  Charleston, 
S.  C.  Few  known  earthquakes  anywhere  have  shaken  a  larger 
area,  and  hence  the  jar  which  caused  the  Charleston  earthquake 
must  have  been  among  the  most  energetic  of  which  the  world  has 
record;  and  yet  many  earthquakes  have  been  much  more  violent. 
Hence  its  origin  must  have  been  one  of  the  most  deeply  buried. 
The  boundary  of  its  epicentral  district  was  well  marked,  and  from 
it  the  depth  of  the  origin  was  calculated  to  be  about  12  miles. 
Within  the  epicentral  district  was  the  little  collection  of  frame  build- 
ings, called  Summerville.  This  was  terribly  shaken,  and  a  dozen  or 
more  of  its  wooden  houses  were  wrecked.  In  Charleston  almost  all 
the  brick  buildings  were  severely  injured,  and  a  large  number  com- 
pletely wrecked.  Many  chimneys  were  overthrown  as  far  distant  as 
Atlanta,  Ga.,  (250  miles),  Asheville,  N.  C,  (230  miles),  and  Raleigh, 
N.  C,  (215  miles).  Had  the  epicentrum  occurred  a  few  miles 
farther  south-east,  or  had  the  city  been  underlaid  by  a  less  depth  of 
loosely  compacted  strata,  Charleston  would  probably  have  been  laid 
in  ruins,  and  the  loss  of  life  would  have  been  vastly  greater  than  it 
was. 

In  May,  1887,  an  earthquake  shook  the  region  between  the  Colo- 
rado and  the  Rio  Grande  from  Utah  almost  as  far  south  as  the  city 
of  Mexico.  Its  epicentral  district,  in  the  Mexican  state  of  Sonora, 
included  the  town  of  Babispe,  which  was  entirely  destroyed.  The 
epicentral  district  was  found  to  be  traversed  by  a  new  fault  35  miles 
long,  of  which  the  vertical  displacement  averaged  8  feet. 


CHAPTER  XX. 

VOLCANOES. 

Bow  thy  heavens,  O  Lord,  and  come  down :  touch  the  mountains,  and  they  shall 
smoke.— Psalm  cxliv:  5 

A  volcano  is  essentially  a  collection  of  ducts  or  fissures 
in  the  earth,  from  which  intensely  hot  gases  and  rocky 
material  have  been  discharged.  The  rocky  material  dis- 
charged usually  accumulates  around  the  ducts  into  a  more 
or  less  isolated  and  cone-shaped  heap  called  a  volcanic  cone, 
which  may  reach  an  altitude  of  many  thousand  feet,  and 
cover  an  area  of  hundreds  or  even  thousands  of  square 
miles.  The  principal  mouth,  or  vent,  of  a  volcano  usu- 
ally occurs  in, a  hollow,  called  the  crater,  near  the  summit 
of  the  cone. 

Volcanic  eruptions  vary  greatly  in  intensity  at  differ- 
ent times  and  places.  A  few  volcanoes  are  constantly 
discharging  matter;  usually,  however,  volcanic  activity  is 
intermittent,  —  eruptions  lasting  days,  weeks,  or  months, 
alternating  with  dormant  periods,  lasting  years  or  even 
centuries,  during  which  there  is  no  discharge.  Continu- 
ous eruptions,  or  those  recurring  at  short  intervals,  are 
seldom  very  violent ;  violent  eruptions  generally  succeed, 
and  are  followed  by,  proportionately  long  periods  of  rest. 
Eventually,  after  perhaps  thousands  of  years  of  such  con- 
stant or  intermittent  activity,  the  great  heat  beneath  a 
vent  subsides  permanently,  and  the  volcano  becomes  ex- 
tinct. 

(•78) 


VOLCANOES.  2  7Q 

The  materials  discharged  in  eruptions  are  chiefly 
melted  rock  or  lava  and  steam.  When  the  lava  rises  in  the 
ducts,  its  entire  mass  seems  to  be  permeated  with  steam, 
which  escapes  from  it  more  or  less  explosively.  Rela- 
tively small  quantities  of  other  gases  are  generated  by  the 
heat  from  various  minerals  in  the  lava,  and,  by  their  action 
upon  each  other  and  the  surface  rocks,  frequently  cause 
deposits  of  sulphur,  alum,  gypsum,  salt,  and  other  sub- 
stances to  accumulate  about  the  volcanic  vent.  Some  of 
these  gases  are  combustible,  and  are  ignited  by  the  heat ; 
but  the  flames  are  only  feebly  luminous,  and  never  form 
a  conspicuous  feature  of  an  eruption. 

The  lava  is  discharged  both  in  streams  and  in  frag- 
ments, the  proportion  discharged  in  either  manner  de- 
pending largely  upon  the  violence  of  the  eruption.  In 
very  quiet  eruptions,  the  lava  is  discharged  chiefly  in 
streams,  while  in  some  very  violent  eruptions  it  is  entirely 
ejected  in  fragments ;  usually,  however,  lava  is  ejected  in 
both  ways  during  an  eruption. 

The  violence  of  an  eruption  depends  to  a  great  ex- 
tent upon  the  fluidity  of  the  lava  and  the  abundance  of 
its  permeating  steam.  With  stiff  lava  in  the  ducts,  the 
steam,  when  abundant,  escapes  spasmodically  with  terrific 
explosions,  hurling  to  prodigious  heights  and  distances 
vast  quantities  of  glowing  lava  masses,  and  blocks  of  rock 
torn  from  the  crater  or  the  sides  of  the  duct.  The  lava 
masses  are  of  various  shapes  and  sizes,  and,  cooling  in 
the  air,  fall  as  globular  bombs;  jagged  and  slag-like 
cinders  or  sconce ;  glassy  and  bubble-impregnated  pumice ; 
gravelly  lapelli ;  sand;  and  the  fine,  glassy  dust  called 
volcanic  ashes.  Deluges  of  rain  from  the  condensing 
steam  falling  on  the  cone  transform  the  dust  into  a  fine 
fluid  mud,  which  hardens  into  a  compact  rock  called  tuff, 
while  the  larger   fragments   are   cemented   together   into 


28o  PHYSICAL    GEOGRAPHY. 

volcanic  conglomerate.  The  steam  escapes  more  readily 
and  continuously  from  very  fluid  lava ;  hence,  a  violent 
eruption  of  such  lava  seldom  occurs.  But  even  very  fluid 
lava  is  viscous,  like  syrup,  and  the  escaping  vapors  carry 
up  from  its  surface  long  filaments,  which,  when  cool,  re- 
semble spun  glass.  This  is  called  Pele's  Hair  in  Hawaii, 
where  it  is  formed  in  great  quantities. 

The  fluidity  of  lava  depends  largely  upon  its  mineral  composi- 
tion. When  composed  largely  of  infusible  silica  it  is  called  trachytic 
lava,  which  is  never  thoroughly  melted,  and  is  always  stiff.  When 
less  silica  is  present,  it  is  called  basaltic  lava.  This  melts  more 
readily,  becoming  as  fluid  as  melted  glass,  and  is  apt  to  resemble 
glass  upon  cooling  rapidly. 

Lava  streams  issue  at  a  white  heat  either  over  the 
edge  of  the  crater,  or  more  frequently  from  fissures  in  the 
side  of  the  cone,  and  flow  rapidly  at  first.  Very  soon 
a  cool,  solid  crust  forms  over  a  stream,  which  moves  very 
slowly,  while  the  interior,  prevented  by  the  non-conduct- 
ing crust  from  cooling  quickly,  flows  faster,  and  constantly 
bursts  through  the  cool  crust  that  rapidly  forms  on  the 
front  of  the  stream.  The  flow  of  the  interior  sometimes 
leaves  long  hollows  or  tunnels  beneath  the  crust,  but  usu- 
ally the  slow  advance  and  contraction  of  the  cooling  crust 
break  it  up  into  countless  blocks  which  settle  down  into 
the  cavities  beneath  (Fig.    114). 

The  crust  of  a  lava  stream  is  such  a  poor  conductor  that  the 
interior  may  remain  at  a  red  heat  for  many  months  after  its  eruption, 
and  during  this  time  the  whole  mass  may  be  imperceptibly  advanc- 
ing. Enormous  volumes  of  steam  escape  through  the  crevices  in 
the  cool  crust  of  a  fresh  lava  stream  from  the  hot  interior,  some- 
times throwing  up  miniature  cones.  The  steaming  vents  on  a  lava 
stream  are  called  spiracles  or  fumaroles.  The  length  of  lava  streams 
depends  chiefly  upon  the  amount  of  lava  erupted  and  its  liquidity. 
Streams  from  1  to  5  miles  long  are  very  common,  but  the  enormous 
outflows  of  very  liquid  lava  in  Hawaii  and  Iceland  have  reached 
distances  of  from  30  to  50  miles. 


VOLCANOES. 


28l 


Fig.  114. — An  old  Lava  Stream  on  Vesuvius. 


General  Shape  of  Cones. — The  material  ejected  is 
deposited  over  a  wide  area,  but  in  decreasing  quantities  as 
the  distance  from  the  vent  increases;  thus,  the  conical 
shaped  heap  or  mountain  is  gradually  built  up  by  the  de- 
posits of  successive  eruptions.  The  steepness  of  volcanic 
cones  varies  greatly,  and  depends  partly  upon  the  average 
liquidity  of  the  lava  in  its  various  eruptions.  Cones  built 
chiefly  of  fragmental  material  are  apt  to  be  steep,  since 
this  material  will  stand  at  a  slope  of  about  35°.  Outflows 
of  stiff  lava  also  produce  steep  slopes.  Very  fluent  lava, 
on  the  contrary,  is  apt  to  produce  flatter  cones ;  those  of 
Hawaii  and  Iceland  have  an  inclination  of  less  than  io°, 
while  in  many  parts  of  the  world,  as  in  the  western  part 
of  the  United  States,  the  peninsula  of  India,  the  plateau 
of  Abyssinia,  and  elsewhere,  tens  of  thousands  of  square 
miles  are  covered  many  hundreds  or  even  thousands  of 
feet  deep  under  successive  outflows  of  ancient  basaltic  lava 
in  practically  horizontal  layers.     These  lavas  must  have 


282  PHYSICAL    GEOGRAPHY. 

been  very  fluid  at  the  time  of  their  emission,  since  no  cone 
at  all  seems  to  have  been  produced.  It  is  even  thought 
that  they  may  have  welled  quietly  up  through  numerous 
long  fissures  over  the  several  regions  without  many  of  the 
phenomena  characteristic  of  modern  volcanoes. 

The  internal  structure  of  many  cones  has  been  laid 
bare  by  prolonged  erosion  after  the  volcano  has  become 
extinct.  The  fragmental  deposits  and  lava  streams  of  suc- 
cessive eruptions  give  the  cone  an  irregularly  stratified 
structure,  the  strata  having  a  general  dip  away  from  the 
central   ducts.     These   inclined   beds  are   intersected    by 


W/S\   TUFF     *     FRAGMENTS 

F=*=3   LAVA 

I  '|  ■  i1  I   ROCK     BENEATH     CONE 


Fig.  115.— Ideal  Section  of  a  Volcano. 

numerous  more  or  less  vertical  dikes,  which  radiate  in 
all  directions  from  the  central  ducts,  and  which  are  simply 
great  lava-filled  fissures  which  have  been  rent  in  the  cone 
during  eruptions.  Some  of  these  fissures  reach  to  the 
surface  of  the  cone,  and  such  are  the  source  of  most  lava 
streams ;  others  do  not  open  through  to  the  surface,  and 
appear  as  dikes  only  after  erosion  has  worn  away  the  over- 
lying beds.  A  great  fissure  in  a  large  cone  is  often 
marked  by  a  line  of  minor  or  parasitic  cones,  thrown  up 
successively  along  its  course  as  it  opens.  The  great  cone 
of  Etna  has  more  than  200  parasitic  cones,  some  of  them 


f^aej ?  Stratified  rock  aui<xy 

■Mi  ■  Lava  ^^-''^ 

Y\y%    MILES   HIGH  yflO^' 


VOLCANOES.  283 

_6y— erosion  -  ^ 


Fig.  116. — Laccolite  forming  Mt.  Hillers,  Henry  Mountains. 

over  600  feet  high.  In  some  cases,  the  rocky  platform  be- 
neath has  subsided  to  a  greater  or  less  extent  as  the  cone 
accumulated.  In  other  cases,  the  ascent  of  stiff  trachytic 
lava  in  the  central  ducts  seems  to  have  bent  the  adjacent 
strata  upward,  and  very  frequently  it  is  found  to  have 
penetrated  horizontally  between  strata,  forming  deeply 
buried  horizontal  sheets  of  great  extent. 

These  intrusions  of  trachytic  lava  between  deeply  buried  strata 
are  not  only  very  extensive  but  sometimes  very  thick,  when  they 
are  called  laccolites.  Their  formation  sometimes  pushes  up  the 
overlying  strata  to  form  great  dome-like  hills  or  mountains  at  the 
earth's  surface.  The  Henry  Mountains,  an  isolated  group  in 
southern  Utah,  seem  to  have  been  upheaved  by  such  a  subterranean 
intrusion  of  lava,  which,  instead  of  reaching  the  surface  as  an  ordi- 
nary volcano,  spread  out  between  the  strata  at  a  depth  of  between 
2  and  3  miles,  forming  25  or  more  great  circular  laccolites  or  lava 
cakes,  the  largest  of  which  is  about  4  miles  in  diameter  and  \% 
miles  thick.  The  overlying  strata,  pushed  up  into  domes  by  these 
laccolites,  have  been  in  places  entirely  removed  by  ages  of  subse- 
quent erosion,  thus  uncovering  portions  of  the  laccolites  and  reveal- 
ing the  cause  of  the  uplift  (Fig.  116).  In  Colorado,  New  Mexico, 
and  Arizona,  as  well  as  in  foreign  countries,  trachytic  lava  in  great 
masses  has  been  partially  uncovered  by  prolonged  erosion ;  many 
of  these  masses  are  doubtless  laccolites,  and  indicate  that  such  sub- 
terranean intrusions  are  by  no  means  exceptional. 


284  PHYSICAL   GEOGRAPHY. 

Changes  in  the  Crater. — Every  eruption  changes  the 
shape  or  size  of  a  cone.  Minor  fragmental  eruptions  in- 
crease its  height  and  bulk,  but  great  or  violent  eruptions 
generally  decrease  its  height,  for  the  whole  top  of  a  cone 
may  be  shattered  and  blown  off  in  fragments  by  the  vio- 
lent explosions,  or  it  may  be  engulfed  when  a  copious  dis- 
charge of  lava  drains  away  its  subterranean  liquid  support. 
Thus,  great  eruptions  often  transform  the  upper  part  of  a 
£one  into  a  huge  abyss,  called  a  caldera,  which  may  be 
several  miles  in  diameter,  and  more  or  less  completely  sur- 
rounded by  precipitous  cliffs  thousands  of  feet  high. 
Minor  eruptions  build  up  a  new  cone  within  a  caldera, 
which  may  fill  and  obliterate  it  before  an  eruption  again 
occurs  of  sufficient  energy  to  destroy  the  top  of  the  cone. 

The  crater  of  Kilauea  (Hawaiian  Islands)  is  a  caldera  3  miles 
long  and  2  miles  wide.  It  almost  always  contains  pools  of  liquid 
lava.  The  pools  constantly  overflow  ;  successive  overflows,  cooling, 
gradually  build  up  the  bottom  of  the  pit,  until  suddenly,  after  an 
indefinite  interval  of  years,  subterranean  fissures  open,  through 
which  the  lava  pools  drain  away,  and  the  bottom  of  the  caldera 
sinks,  while  great  slices  often  fall  from  the  precipitous  sides  into  the 
abyss  and  thus  increase  its  area.  Old  craters  and  calderas  often 
become  filled  with  water.  Such  lakes  are  common  in  all  volcanic 
districts.  Lake  Taupo,  in  New  Zealand,  is  thought  to  occupy  one 
of  the  largest  calderas  in  the  world.  The  lake  is  20  miles  in  diam- 
eter, and  is  surrounded  by  cliffs  1,000  feet  high.  Crater  Lake,  in 
the  Cascade  Mountains  of  Oregon,  occupies  another  caldera  7% 
miles  long  and  5  miles  wide.  The  lake  is  2,000  feet  deep,  and  is 
completely  encircled  by  cliffs  1,000  to  2,000  feet  high.  From  its 
surface  an  extinct  cinder  cone  600  feet  high  rises  as  an  island,  bear- 
ing a  perfect  crater  in  its  summit. 

Eruptions. — Violent  eruptions  are  usually  preceded  by 
muffled  noises  and  earth  tremors  or  shocks,  caused  proba- 
bly by  the  fracturing  of  subterranean  strata.  Then  follow 
explosions  which  occasion  heavy  local  earthquakes.  The 
crater  breaks  up,  and  solid  blocks  and  glowing  lava  frag- 
ments are  scattered  far  and  wide,  while  the  steam  escaping 


VOLCANOES.  285 

at  each  explosion,  rising  rapidly  and  condensing,  adds  a 
great  globular  mass  to  the  dust  and  cloud  canopy  forming 
above  (page  40).  This  canopy  reflects  the  glow  of  the 
liquid  lava  in  the  ducts,  and,  together  with  the  rapid 
ascent  of  incandescent  fragments,  produces  the  illusion  of 
brilliant  tongues  of  flame  issuing  from  the  crater.  The 
column  rising  from  the  crater  often  reaches  a  height  of 
several  miles,  within  which  is  generated  electricity,  mani- 
fested by  incessant  flashes  of  lightning  and  terrific  peals 
of  thunder.  Rainbows  and  halos  are  produced  by  the 
play  of  light  through  the  water  globules  of  the  condensing 
steam,  while  the  violent  local  updraught  in  the  atmos- 
phere generally  occasions  terrific  winds  in  the  district  sur- 
rounding the  volcano.  With  an  outflow  of  lava  an 
eruption  subsides,  though  sand  and  dust  continue  for 
some  time  to  be  discharged  to  great  heights  and  in  such 
quantities  as  often  to  exclude  all  daylight  from  a  great 
extent  of  the  surrounding  country.  Eventually  the  dis- 
charge of  all  solid  matter  ceases,  but  steam  and  gases 
continue  for  a  long  period  to  rise  from  crevices  in  the 
cone  and  from  the  lava  streams.  Quiet  eruptions  may  or 
may  not  occasion  earthquakes,  and  may  consist  simply  of 
the  issue  of  steaming  lava  streams  from  the  side  of  the 
cone.  This  is  usually  preceded  by  a  rise  of  lava  into  the 
crater,   and  an  increased  discharge  of  steam. 

The  enormous  energy  of  volcanic  action  is  most  strikingly  dis- 
played in  the  infrequent  but  very  violent  eruptions.  Thus,  in  a 
single  night  of  181 5  the  top  was  blown  from  Tomboro,  in  the  Malay 
Archipelago,  reducing  its  cone  from  a  shapely  peak  2  miles  high  to 
a  mere  stump,  less  than  half  as  high,  with  a  huge  caldera  in  the 
top.  The  eruption  of  Krakatoa,  in  1883,  was  another  instance  of 
excessively  violent  volcanic  action.  Its  explosions  were  audible  for 
2,000  miles  in  all  directions,  or  over  ^th  of  the  earth's  surface,  and 
a  perceptible  layer  of  the  dust  ejected  fell  at  all  places  within  1,000 
miles  of  the  volcano ;  while  the  finest  dust  and  vapor,  shot  up  1 5  or 
16  miles  high,  were  generally  distributed  over  the  globe,  causing, 


286  PHYSICAL    GEOGRAPHY. 

while  still  suspended  in  the  atmosphere,  the  peculiar  red  sunsets 
noticed  in  all  parts  of  the  world  for  months  after  the  eruption  (p.  104). 
The  volcanoes  of  Hawaii  often  exude  lava  streams  which  cover  100 
to  200  square  miles  to  a  depth  of  100  feet  or  more ;  but  they  are  dis- 
charged so  quietly  that  the  display  of  energy  is  not  striking.  Re- 
peated outflows  of  this  kind,  however,  during  untold  ages,  have 
built  up  a  great  flat  cone  6  miles  high  from  the  ocean  floor,  to  form 
the  lofty  island  which  is  half  as  large  as  New  Jersey.  This  cone 
must  contain  material  enough  to  cover  the  whole  United  States  50 
feet  deep,  and  the  energy  required  to  heap  it  up  is  probably  as  great 
in  the  aggregate  as  that  displayed  during  the  life  of  any  violently 
active  volcano  in  the  world. 

Gradual  Decay  of  Volcanic  Activity.  —  The  cool- 
ing of  the  earth's  crust  beneath  an  old  volcanic  region 
is  an  exceedingly  slow  process.  For  ages  after  all  other 
signs  of  activity  have  ceased,  steam  and  volcanic  gases 
continue  to  escape  at  some  volcanoes  from  the  numerous 
fissures.  A  volcano  in  this  condition  is  said  to  be  in  the 
solfatara  stage.  Gradually,  the  heat  in  the  superficial 
parts  of  the  crust  subsides  until  no  longer  great  enough  to 
convert  all  of  the  percolating  water  into  steam,  and  the 
old  volcanic  region  becomes  a  district  of  hot  springs. 

Most  of  the  warm  springs  in  the  world,  and  nearly  all  the  very 
hot  ones,  occur  in  or  near  volcanic  formations,  though  frequently  in 
localities  where  no  volcanic  eruption  has  taken  place  for  hundreds, 
and  probably  for  many  thousands  of  years. 

Geysers. — The  hot  springs  of  volcanic  regions  are 
characterized  not  only  by  their  high  temperature,  but  by 
the  immense  quantities  of  mineral  matter,  usually  silica, 
which  they  bring  to  che  surface  and  deposit  over  their 
neighborhood  in  fantastic  and  intricate  forms.  Often  the 
deposit  forms  extensive  terraces  of  silicious  sinter  through 
which  the  streams  rise  into  deep,  funnel-like  basins.  If 
the  water  enters  such  a  basin  slightly  above  its  boiling 
temperature,  the  spring  may  become  a  geyser  (spouter  or 
gusher). 


VOLCANOES.  287 

The  water  near  the  surface,  chilled  by  the  air,  is  kept  beneath  its 
boiling  temperature,  while  the  water  below  is  kept  from  boiling  by 
the  pressure  of  that  above.  Thus,  the  lower  water  becomes  super- 
heated, and  gradually  heats  the  surface  water,  which  at  last  begins 
to  boil.  This  relieves  the  pressure  on  the  water  immediately  below, 
which,  being  above  its  boiling  point,  vaporizes  explosively,  and 
forces  into  the  air  a  cloud  of  steam  and  a  jet  of  the  overlying  water. 
This  considerable  relief  from  pressure  is  followed  by  louder  explo- 
sions in  the  still  hotter  water  beneath,  and  the  more  violent  dis- 
charge of  water  jets  and  steam  clouds  into  the  air.  The  explosions 
and  discharges  continue  until  the  basin  is  emptied  and  the  water  in 
the  conduits  is  chilled  below  its  boiling  point  by  exposure  to  the  air. 
The  eruption  then  ceases,  and  the  water  rises  quietly  in  the  basin 
until  the  conditions  are  suitable  for  another  eruption.  The  eruptions 
of  a  geyser  occur  at  more  or  less  regular  intervals  of  time,  but  these 
intervals  vary  in  different  geysers,  from  a  few  minutes  to  many 
hours  or  days.  By  the  continued  mineral  deposit,  the  shape  and 
dimensions  of  a  basin  may  be  so  changed  as  to  convert  a  hot 
spring  into  a  geyser,  or  a  geyser  into  an  ordinary  hot  spring. 

Geysers  occur  in  many  volcanic  districts  oyer  the  world.  They 
are  most  numerous  and  powerful  at  the  sources  of  the  Missouri  in 
Yellowstone  Park,  Wyoming ;  near  Mount  Hecla,  in  Iceland ;  and 
in  the  North  Island  of  New  Zealand  •  but  are  also  found  in  Mexico, 
the  West  Indies,  the  Azores,  Thibet,  the  Malay  Archipelago,  the  Fiji 
Islands,  and  possibly  other  places.  The  so-called  geysers  of  Cali- 
fornia and  Nevada  are  violently  boiling  springs  rather  than  true 
periodic  geysers,  though  they  are  closely  associated  phenomena. 
In  Yellowstone  Park  there  are  more  than  3,000  hot  or  boiling  springs, 
including  71  geysers,  of  which  the  most  noted  are:  the  Giantess, 
which  throws  jets  250  feet  high,  at  intervals  of  several  weeks;  the 
Bee  Hive,  spouting  219  feet,  at  intervals  of  14  to  16  hours;  Grand 
Geyser,  200  feet,  at  intervals  of  16  to  30  hours ;  the  Giant  and  Castle 
geysers,  spouting  about  200  feet  high,  and  Old  Faithful  (see  frontis- 
piece), which  every  hour  throws  up  jets  to  a  height  of  about  150 
feet.  Whenever  hot  springs  occur  in  clay  formations,  the  water  in 
the  basin  is  apt  to  become  more  or  less  muddy  from  repeated  caving 
in  of  the  banks.  Sometimes  the  pool  thus  acquires  a  thick,  por- 
ridge-like consistency,  and  if  the  temperature  be  high  enough  to 
cause  the  water  to  boil,  the  explosion  of  steam-bubbles  beneath  the 
surface  scatters  the  mud  about.  Such  mud  springs  are  called  mud 
volcanoes.    They  are  common  in  all  hot-spring  districts. 


288  PHYSICAL    GEOGRAPHY. 

Distribution. — The  indications  of  past  or  present  vol- 
canic action  are  found  in  all  latitudes  and  longitudes,  and 
at  all  elevations.  They  occur  on  the  continents,  and  on 
both  the  continental  and  oceanic  islands,  while  several  re- 
corded submarine  eruptions  attest  their  occurrence  upon 
the  sea  bottom.  Volcanic  activity  seems  to  have  been 
present  somewhere  on  the  earth's  surface  throughout  geo- 
logical time,  but  has  gradually  shifted  the  site  of  its 
activity  to  new  areas  during  the  long  course  of  the  world's 
history.  The  total  number  of  localities  in  the  world  in 
which  are  found  indications  of  volcanic  action,  ancient  or 
modern,  would  reach  tens  of  thousands.  About  300  vol- 
canoes are  known  to  be  active.  About  one  half  of  these 
occur  on  the  continental  islands  lying  south-east  and  east 
of  Asia,  and  extending  from  New  Zealand  to  Alaska. 
Within  this  region,  volcanism  is  at  present  more  energetic 
than  elsewhere  on  the  globe.  About  one  fourth  of  the 
active  volcanoes  are  distributed  irregularly  along  the  ele- 
vated western  margin  of  the  American  main-land,  from 
Alaska  to  the  Str.  of  Magellan.  There  are  about  45 
active  vents  in  North  and  Central  America,  and  about  37 
along  the  Andes.  A  few  have  been  discovered  in  east 
Africa,  and  on  islands  along  that  coast.  Thus,  fully  three 
fourths  of  all  active  volcanoes  known  lie  just  within  the 
convex  or  generally  rising  border  of  the  continental 
plateau.  Only  one  eighth  of  the  world's  active  volcanoes 
occur  elsewhere  on  the  continental  plateau,  and  these  are 
found  in  widely  separated  groups.  Three  of  these  groups — 
Iceland,  with  1 3  active  vents ;  the  Lesser  Antilles,  with  6, 
and  the  Canary  Islands,  with  3 — occur  in  rising  localities 
on  the  generally  subsiding  concave  margin  of  the  plateau ; 
a  fourth  group  of  7  vents  occurs  in  a  rising  area  on  the 
margin  of  the  deep  Mediterranean  depression,  while  the 
rest,    5   or  6  in  number,   are   found   along  the  northern 


(289) 


290 


PHYSICAL    GEOGRAPHY. 


margin  of  the  geologically  recent  highlands  of  Asia.  The 
remaining  eighth  of  all  the  active  volcanoes  are  distributed 
along  the  great  submarine  ridges  which  traverse  the 
oceanic  depressions.  There  are  about  20  active  vents  in 
the  depression  of  the  Pacific,  10  in  that  of  the  Atlantic, 
and  2  or  3  in  that  of  the  Indian  Ocean,  while  there  are  at 
least  2  within  the  Antarctic  Circle. 


Fig.  117. — Volcanic  Necks  in  western  New  Mexico. 


Indications  of  former  volcanic  action  which  is  now  either 
dormant  or  extinct  are  also  found  in  all  the  regions  mentioned 
above,  and  in  many  other  localities  over  the  land  areas,  chiefly  in 
highly  tilted  and  disturbed  strata,  such  as  are  frequent  in  mountain 
regions ;  in  fact,  almost  every  mountain  range  in  the  world  has 
associated  with  it,  either  in  its  mass  or  near  its  base,  vestiges  of 
volcanic  action.  It  is  generally  true  that  indications  of  very  re- 
cently extinct  action  are  more  numerous  toward  the  convex  side 
of  the  continental  plateau,  while  vestiges  of  very  ancient  and  long 
extinct  volcanism  are  more  numerous  on  the  concave  side.  Indica- 
tions of  very  recent  action  are  found  throughout  the  West,  and  of 
very  ancient  volcanic  action  throughout  the  eastern  part  of  the 
Union.  In  the  Cascade  Range  are  many  great  volcanic  cones  be- 
tween 10,000  and  14,000  feet  high.     Some  of  them  are  still  emitting 


VOLCANOES.  29I 

steam  and  gases.  At  Feather  Lake,  in  northern  California,  a  fresh 
lava  stream  3X  miles  long  and  a  mile  wide  occurs,  and  is  said  to 
have  been  erupted  in  1850.  Fresh  lavas,  possibly  a  century  or  two 
old,  are  found  in  Utah  and  Arizona.  In  western  New  Mexico  are 
lava  streams  24  miles  long  and  4  miles  wide,  which  can  not  be 
many  centuries  old ;  but  in  the  same  neighborhood  are  the  remains 
of  much  older  lava  streams,  which  in  the  tertiary  era  flooded  this 
region  from  many  vents.  Prolonged  erosion  has  completely  re- 
moved most  of  this  old  lava  cap,  and  also  a  great  thickness  of  the 
strata  beneath  it,  leaving,  however,  on  the  site  of  each  vent  an 
isolated  hill  or  mountain  composed  of  the  hard  lava  which  solidified 
in  the  duct  when  the  volcanism  subsided.  Scores  of  such  "  volcanic 
necks,"  from  800  to  1,500  feet  high,  are  found  in  that  vicinity. 
(Fig.  117.)  Indications  of  still  older  volcanic  action  are  found  in 
the  tilted  lava  sheets  which  traverse  the  eastern  part  of  the  Union 
from  Maine  to  South  Carolina.  The  Palisades  of  the  Hudson,  and 
Mt.  Tom  and  Mt.  Holyoke,  of  Massachusetts,  are  such  sheets. 
These  sheets  were  erupted  early  in  the  mesozoic  era,  long  before  the 
Rocky  Mountains  were  upheaved.  The  tilted  lava  sheets  which 
form  Keweenaw  Point  and  the  Gogebic  Range  south  of  Lake 
Superior,  are  still  older.  The  eruption  of  these  sheets  took  place 
before  the  Alleghanies  were  upheaved  —  in  early  paleozoic  times. 

Causes  of  Volcanic  Action. — All  active  volcanoes 
seem  to  occur  in  regions  which  are  rising.  It  is  probable 
that  the  heat  which  melts  subterranean  rock  masses  into 
lava,  and  leads  to  its  ejection,  is  but  a  peculiar  manifesta- 
tion of  the  same  energy  which  causes  the  upheavals  of  the 
earth's  crust.  Whatever  be  the  causes  of  these  move- 
ments, it  seems  certain  that  the  friction  of  the  moving 
rock  particles  against  each  other  would  generate  excep- 
tionally intense  heat  at  certain  places  within  the  rising 
mass.  The  heat  in  these  localities  may  become  great 
enough  to  liquefy  the  more  fusible  rocks  at  a  compara- 
tively slight  depth,  though  not  great  enough  to  liquefy  the 
less  fusible  surrounding  rocks.  Thus,  a  subterranean 
cavity,  or  vesicle,  full  of  molten  lava,  is  formed,  which 
may  be  many  miles  in  horizontal  dimensions,  and  many 

hundred  feet  in  vertical  depth.     When,  owing  to  the  pe- 
p.  G.-17. 


292  PHYSICAL   GEOGRAPHY. 

culiarities  in  mineral  composition  which  determined  its 
fusibility,  the  molten  mass  is  lighter,  bulk  for  bulk,  than 
the  solid  rocks  above,  the  great  weight  of  the  latter  causes 
them  to  sink  down  into  the  cavity,  squeezing  the  molten 
lava  upward  into  the  fissures  caused  by  the  subsidence. 
If  the  difference  in  weight  between  the  lava  and  the  solid 
mass  above  is  but  slight,  the  lava  may  rise  only  part  way 
to  the  surface,  and  spread  out  between  the  strata  to  form 
subterranean  lava  sheets  or  laccolites ;  but  if  the  difference 
in  weight  is  great  enough,  the  lava  is  squeezed  upward  to 
the  surface,   to  overflow  and  form  a  volcano. 

Thus,  steam  is  probably  not  an  essential  factor  in  bringing  the 
lava  up  to  the  earth's  surface,  though  all  lava  seems  to  be  perme- 
ated with  steam  when  it  reaches  the  surface,  and  the  degree  of  vio- 
lence of  volcanic  eruptions  probably  depends  upon  the  manner  in 
which  this  steam  escapes  from  lavas  of  different  mineral  composi- 
tion, and  from  the  same  lava  at  different  temperatures  and  pressures. 
It  seems  probable  that  the  water  percolating  through  all  rocks  is 
converted  at  some  depth  into  steam  by  the  subterranean  heat,  and 
as  such  is  absorbed  or  occluded  by  the  molten  lava  in  very  much 
greater  quantity  than  the  lava  is  able  to  retain  when  its  temperature 
and  pressure  diminish  as  it  rises  toward  the  earth's  surface.  Hence, 
the  excess  of  the  absorbed  steam  escapes,  or  is  excluded,  more  or 
less  explosively,  according  to  the  viscosity  of  the  lava,  producing  a 
more  or  less  violent  eruption.  It  is  a  somewhat  similar  exclusion 
of  carbonic  acid  gas,  absorbed  by  water  under  high  pressure,  that 
produces  the  effervescence  of  soda  water  when  the  pressure  within 
the  "fountain"  is  relieved,  by  opening  the  nozzle. 


PART  V. — WEATHER  AND  CLIMATE. 


CHAPTER  XXI. 
WEATHER   AND   CLIMATE. 


When  it  is  evening,  ye  say,  It  will  be  fair  weather:  for  the  sky  is  red.  And 
in  the  morning,  It  will  be  foul  weather  to  day  :  for  the  sky  is  red  and  lowering. — 
Matthew  xvi  :  2,  3. 

Weather  is  the  condition  of  the  atmosphere  at  any 
time  and  place  with  respect  chiefly  to  its  temperature, 
humidity,  clearness  or  cloudiness,  rain,  fog,  or  snow,  and 
wind. 

Changes  of  Weather. — The  weather  is  every-where 
constantly  changing,  owing  to  the  diurnal  and  seasonal 
variations  of  temperature.  But,  in  addition  to  these  com- 
paratively regular  changes,  others,  much  less  regular,  take 
place  as  a  result  of  the  passage  of  cyclonic  winds  or 
storms,  which  may  quickly  replace  the  air  over  any 
locality  with  other  air  having  a  very  different  temperature 
and  humidity. 

In  the  torrid  zone  cyclones  seldom  occur,  excepting 
in  the  western  part  of  the  tropical  oceans;  and  hence  the 
weather- changes  in  that  zone,  depending  principally  upon 
the  variations  in  the  position  of  the  sun,  occur  with  great 
regularity,  the  same  changes  often  taking  place  at  the 
same  hour,  day  after  day,  for  weeks  together,  every  year. 

(293) 


294  PHYSICAL    GEOGRAPHY. 

In  temperate  zones,  cyclonic  winds  are  much  more 
common.  Between  latitudes  400  and  700,  where  they  are 
of  most  frequent  occurrence,  an  endless  procession  of 
cyclones  and  anticyclones  moves  eastward.  Though  their 
general  movement  is  easterly,  the  different  whirls  seldom 
move  in  exactly  the  same  direction  (see  chart,  page  91) 
or  at  the  same  rate  of  speed ;  hence,  different  ones  pass 
over  the  same  locality  at  irregular  intervals.  Each  whirl 
produces  variations  of  weather  as  it  passes  over  a  locality, 
which  modify  in  a  marked  degree  those  regular  variations 
due  to  the  alterations  in  the  relative  position  of  the  sun; 
and  as  the  whirls  arrive  at  irregular  intervals,  the  weather- 
changes  are  as  irregular  in  temperate  latitudes  as  they  are 
regular  in  equatorial  regions. 

Weather  Probabilities. — From  long  observation  of 
the  paths  traveled  by  cyclones  and  anticyclones  under 
different  circumstances,  the  officers  of  the  United  States 
Weather  Bureau  are  enabled  to  estimate  with  some  accu- 
racy the  course  which  any  cyclone  or  anticyclone  observed 
in  or  near  the  United  States,  will  pursue  during  the  en- 
suing 24  or  36  hours ;  and  it  is  upon  this  estimate  that 
the  weather  predictions  are  based  which  the  Weather 
Bureau  furnishes  for  publication  throughout  the  Union 
every  morning. 

The  use  of  the  telegraph  in  weather  prediction  began  with  its 
extension  over  this  country  in  1844-48,  but  was  first  systematically- 
done  by  Prof.  Joseph  Henry  and  Prof.  J.  P.  Espy  about  1850.  It 
was  begun  in  Europe  in  1854,  and  after  the  war  was  revived  by  the 
Cincinnati  Chamber  of  Commerce  for  mercantile  purposes.  From 
this  followed  the  action  of  Congress  authorizing  storm  and  flood 
predictions  to  be  made  at  first  by  the  Signal  Service  of  the  Army,  but 
at  present  by  the  Weather  Bureau.  The  atmospheric  pressure  and 
the  condition  of  the  weather  are  carefully  observed  twice  a  day  at  the 
same  moment  of  time  in  all  parts  of  the  country,  and  the  results  are 
telegraphed  to  the  central  office  at  Washington.  Here  the  data  are 
entered  upon  a  map,  the  isobars  drawn,  and  the  successive  positions 


WEATHER  AND    CLIMATE.  295 

of  cyclones  and  anticyclones,  as  they  travel  over  the  country,  thus 
indicated.  It  has  been  found  that  the  topography  of  the  country, 
the  sunshine,  and  the  relative  temperature  and  moisture  in  adja- 
cent cyclones  and  anticyclones,  modify  the  direction  and  speed 
of  movement  of  each;  but  that  in  general  the  centers  of  cyclones 
move  north-eastward  over  the  United  States,  while  anticyclones 
move  south-eastward.  Owing,  however,  to  the  ever-changing  con- 
ditions in  adjacent  whirls,  it  is  usually  impossible  to  predict  with 
any  degree  of  accuracy,  the  course  of  any  observed  cyclone  or 
anticyclone  more  than  24  or  36  hours  in  advance. 

Since  the  wind  whirls  about  the  center  of  all 
cyclones  in  the  same  hemisphere  in  the  same  direction, 
the  weather  on  corresponding  sides  of  all  cyclones  is  very 
similar,  and  the  same  is  true  of  all  anticyclones.  Thus,  in 
the  northern  hemisphere  the  winds  in  the  eastern  part  of 
a  cyclone  come  from  the  south,  and  are  relatively  warm; 
and  as  they  advance  into  colder  latitudes  their  vapor  con- 
denses into  cloud,  rain,  or  snow;  while  in  the  western 
part  of  cyclones  the  winds  come  from  the  north,  are  rela- 
tively cold,  and  as  they  enter  warmer  latitudes  less  con- 
densation takes  place.  Hence,  as  a  cyclone  approaches  a 
place  from  the  west,  relatively  warm,  cloudy,  rainy,  or 
snowy  weather  prevails ;  but  as  the  center  passes  to  east- 
ward over  the  place,  a  change  to  relatively  cold,  clear 
weather  takes  place.  Anticyclones,  on  account  of  the  re- 
versed direction  of  the  whirl,  have  colder  and  clearer 
weather  on  their  east  than  on  their  west  sides;  but  as  the 
air  in  an  anticyclone  is  sinking,  and  hence  becoming 
warmer,  it  frequently  happens  that  little  or  no  condensa- 
tion into  cloud  or  rain  occurs  on  either  of  its  sides. 

The  chart  (Fig.  118)  indicates  the  observed  weather  east  of  the 
Rocky  Mountains  one  November  morning.  A  large  cyclone  is 
central  over  Iowa  (low).  To  the  east  of  low  the  winds  of  the 
whirl  blow  from  south  or  south-east;  to  the  north  of  low,  from 
east  or  north-east;  to  the  west  of  low,  from  north  or  north-west; 
and  to  the  south  of  low.  from  west  or  north-west.     To  the  east  of 


296 


PHYSICAL    GEOGRAPHY. 


* -^Isobars,  euery  Vto^  of  an  inch 

Isotherms,  "     10  degrees 

"Lou>"=  Center  of  CyvJone 
"High"-  Anticyclones 

^ > 


Dlreotion  of  wind  &  clear  weather 
>t        m     it     11   cloudy    11 
«»        a     11     a    rain 


path   of  Cyclone 
Fig.  118. 


LOW,  the  southerly  winds  carry  the  warm  air  northward,  so  that  the 
isotherm  of  400  lies  in  the  latitude  of  Cape  Cod,  Lake  Erie,  and 
southern  Lake  Michigan ;  to  the  west  of  low,  the  northerly  winds 
carry  the  cold  air  south,  so  that  this  same  isotherm  lies  near  the 
coast  of  Texas.  To  the  east  of  low,  the  warm  air  is  constantly 
getting  colder  as  it  moves  northward,  and  its  vapor  condenses,  first 
into  clouds  near  the  south  and  east  edge  of  the  cyclone,  then  into 
rain  as  it  reaches  colder  latitudes,  and  at  last  into  snow  as  its  tem- 
perature falls  below  the  freezing  point.  Close  to  the  west  of  low, 
the  cold  air  from  the  north-west  lowers  the  temperature,  and  the 
vapor  still  remaining  in  the  winds  from  the  north-east,  is  condensed 
into  snow ;  but  some  distance  to  the  west  of  low,  cold  and  clear 
weather  prevails.  This  general  distribution  of  the  various  kinds  of 
weather  over  the  Central  States  was  predicted  24  hours  previous, 
when  the  center  of  the  cyclone  was  observed  to  be  in  Indian 
Territory,  near  the  feather  end  of  the  long  dotted  arrow;  and  its 
present  position  in  Iowa  enabled  the  Signal  Service  to  predict  the 


WEATHER  AND    CLIMATE.  297 

distribution  of  weather  which  prevailed  24  hours  later,  when  the 
cyclone  center  had  advanced  to  the  point  end  of  this  arrow.  The 
decrease  in  pressure  toward  the  extreme  north-west  corner  of  the 
chart  indicates  the  approach  of  a  cyclone  from  that  direction. 
Experience  with  cyclones  in  that  quarter  teaches  that  they  move 
south-east  over  the  Rocky  Mountains  into  Texas  or  Kansas,  and 
thence  north-east  or  east  to  the  Great  Lakes;  and  the  kind  of 
weather  that  their  progress  will  cause  in  various  localities  may  be 
predicted  with  considerable  certainty  at  least  24  hours  in  advance. 

Climate. — If  the  weather  at  any  locality  be  carefully 
observed  for  a  long  time,  it  will  be  found  to  repeat  itself 
more  or  less  exactly,  each  year.  Some  years  may  be  un- 
usually hot  or  dry,  and  others  may  be  exceptionally  cold 
or  wet ;  but  when  many  years  are  compared,  the  general 
similarity  in  the  succession  of  weather  one  year  with 
another,  can  not  fail  to  be  remarked.  This  average  annual 
succession  of  weather  peculiar  to  any  locality,  constitutes 
its  climate.  Climate,  like  weather,  embraces  all  meteoro- 
logical phenomena;  but  the  factors  most  important  to 
agriculture  and  hygiene  are:  (1)  the  mean  annual  temper- 
ature, (2)  the  mean  annual  rain-fall,  and  (3)  the  distribu- 
tion of  sunshine,  temperature,  and  rain-fall  throughout  the 
year.  To  the  navigator  another  factor  of  equal  impor- 
tance is  the  direction  and  force  of  the  wind. 

The  importance  of  the  distribution  of  temperature  and  rain-fall 
through  the  year  appears  from  a  single  example :  San  Francisco 
and  Washington  City  have  the  same  mean  annual  temperature 
(550);  yet  the  Washington  summers  are  180  hotter,  and  the  winters 
180  colder,  —  that  is,  the  annual  variation,  or  range,  of  temperature 
is  360  greater — than  at  San  Francisco,  where  ice  and  snow  in  winter 
and  oppressive  heat  in  summer,  are  alike  unknown.  Sacramento, 
Cal.,  has  only  two  thirds  the  rain-fall  (22  inches)  of  Toledo,  Ohio, 
(33  inches),  and  receives  almost  all  of  it  in  winter  and  spring, 
while  at  Toledo  the  rain-fall  is  nearly  equal  in  each  season,  though 
slightly  greater  in  summer  and  autumn. 

The  latitude  of  a  place  is  the  most  important  factor  in 
connection  with  its  supply  of  heat.     In  consequence  of 


298 


PHYSICAL    GEOGRAPHY. 


m% 

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40% 


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Fig.  119. 


the  increasing  obliquity  of  the  sun's  rays  as  the  poles  are 
approached,  (page  50),  the  mean  annual  heating  power  of 
the  sun's  rays  falling  upon  a  given 
horizontal  area,  decreases  from  the 
equator  to  the  poles  in  about  the 
proportion  indicated  by  the  curved 
line  in  the  diagram  (Fig.  119),  be- 
ing but  xo"ths  as  great  at  the  poles 
as  at  the  equator;  and  hence,  in 
general,  climates  become  colder  as 
one  journeys  away  from  the  equator. 

Effect  of  Latitude  on  Annual  Range  of  Tempera- 
ture.— All  places  receive  heat  by  day  and  lose  heat  by 
night.  At  the  equator  the  days  and  nights  are  always  of 
equal  length ;  hence,  each  night  the  temperature  falls 
about  as  much  as  it  rises  during  the  day,  and  as  the  sun 
at  noon  is  never  very  far  north  or  south  of  the  zenith,  the 
heating  power  of  its  rays  is  nearly  the  same  at  all  seasons. 
Therefore,  the  mean  temperature  of  every  day  in  the  year 
is  nearly  the  same;  consequently,  in  equatorial  regions 
there  is  no  thermal  division  of  the  year  (into  winter  and 
summer),  but  the  climate  over  the  whole  torrid  zone  is 
characterized  by  great  uniformity  of  temperature,  the 
greatest  variation  being,  in  general,  that  between  day  and 
night.  This  is  seldom  more  than  180,  and  at  some  places 
near  the  equator  it  is  much  less. 

At  places  not  on  the  equator  the  lengths  of  the 
days  and  nights  are  constantly  changing ;  for  six  months 
the  days  are  longer  than  the  nights,  and  for  six  months 
the  nights  are  the  longer.  When  the  days  are  the  longer, 
a  place  receives  more  heat  by  day  than  it  loses  during  the 
short  night,  and  thus  accumulating  heat,  its  mean  temper- 
ature rises  for  six  months ;  then,  as  the  nights  become  the 
longer,  it  loses  more  heat  than  it  receives,  and  its  mean 


WEATHER  AND    CLIMATE. 


299 


daily  temperature  falls  for  six  months.  Now,  the  farther 
a  place  is  from  the  equator,  the  greater  is  the  difference 
between  the  length  of  its  days  and  nights  (page  51),  and 
hence  the  greater  is  the  variation  of  temperature  during 
the  year.  In  addition  to  this,  the  sun's  rays,  in  middle 
and  higher  latitudes,  are  much 
more  oblique,  and  their  heat- 
ing power  is  much  less  when 
the  days  are  shortest  than 
when  longest;  and  this  differ- 
ence increases  as  the  distance 
from  the  equator  increases. 
Therefore,  the  climate  in  tem- 
perate and  polar  latitudes  is 
characterized  by  a  great  va- 
riation, or  range,  of  temper- 
ature during  the  year  which 
increases,  in  general,  as  the 
latitude  increases. 


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This  is  graphically  indicated  on  the  diagram  (Fig.  120),  which 
shows  the  average  monthly  mean  temperatures  at : 

latitude    5^°,  Range  40. 

24^°,       "      14°. 

"        43^°.       "      47°. 

8i^°,       "      760. 


Paramaribo, 


Guiana, 
Key  West,  Florida, 
Portland,  Maine, 

Fort  Conger,    Arctic  Regions,  " 


The  diagram  also  indicates  that  in  summer,  when  the  sun  is 
nearly  vertical  over  Key  West,  the  temperature  at  that  place  is 
about  20  higher  than  in  Guiana,  nearer  the  equator.  In  winter, 
however,  when  the  sun  is  over  the  southern  tropic,  the  temperature 
at  Key  West  is  about  io°  lower  than  at  Paramaribo ;  hence,  the 
mean  annual  temperature  is  lower  at  Key  West  than  at  places 
nearer  the  equator,  though  the  summer  temperature  is  higher. 

Effect  of  Land  and  Water  Surfaces  on  Climate. — 
It  has  been  explained  (page  64)  that  a  water  surface  tends 
to  equalize  temperatures,  while  a  land  surface  undergoes 


RANGE 
TEMPERATURE 


(300) 


WEATHER  AND    CLIMATE. 


30I 


extremes  of  heat  and  cold  during  the  year;  and  since  the 
air  acquires  its  temperature  largely  from  the  surface  on 
which  it  rests,  these  peculiarities  are  impressed  upon  the 
climates  of  the  oceans  and  the  land  respectively.  That  is, 
the  climates  of  inland  localities  invariably  have  a  greater 
annual  range  of  temperature  than  those  of  coast  regions, 
or  of  the  open  ocean  in  the  same  latitude. 

Thus,  the  interior  portion  of  the  United  States  from  El  Paso, 
Texas,  to  North  Dakota,  has  an  annual  range  250  to  550 greater  than 
Jhe  Pacific  coast,  and  6°  to  300  greater  than  the  Atlantic  coast  in 
corresponding  latitudes.  The  summers  of  the  interior  are  slightly 
warmer,  but  the  winters  much  colder,  than  those  on  the  coasts  or 
oceans ;  hence,  the  interior  has  a  lower  mean  annual  temperature, 
except  in  equatorial  latitudes,  where,  as  explained  (page  62),  the 
land  is  warmer  than  the  sea  at  all  seasons. 

Continental  and  Oceanic  Climates. — On  account  of 
the  great  influence  of  extensive  land  or  water  surfaces 
upon  the  variations  of  temperature,  localities  at  which  the 
annual  temperature  oscillates  through  a  wide  range  are 
said  to  have  a  continental  climate,  while  those  where  the 
range  is  small  are  said  to  have  an  oceanic  climate. 

The  fitness  of  these  names  is  rendered  apparent  by  the  chart, 
upon  which  the  pink  tint  deepens  as  the  range  (between  the  mean 
temperatures  of  the  hottest  and  coldest  months)  increases.  The 
regions  where  the  range  is  less  than  180  have  no  pink  tint,  and  are 
seen  to  embrace  almost  the  entire  ocean,  except  the  polar  seas, 
where  the  range  is  greater  on  account  of  the  high  latitude.  Almost 
all  the  land  surface,  on  the  contrary,  is  tinted  pink,  and  has  a  range 
greater  than  180.  The  range  increases  inland,  being  about  720  in 
the  interior  of  northern  America,  and  1080  in  the  more  extensive 
grand  division  of  Euro-Asia.  The  only  parts  of  the  land  where  the 
range  is  less  than  180  are  certain  coast  regions,  where  the  influence 
of  the  neighboring  ocean  is  great,  and  the  equatorial  regions  of 
the  land  where,  it  has  been  seen,  the  mean  temperature  of  all  the 
months  is  nearly  the  same;  but  even  here  the  range  between  day 
and  night  sometimes  greatly  exceeds  180  in  the  interior  of  the  con- 
tinents. 


302  PHYSICAL    GEOGRAPHY. 

Climatic  Differences  of  East  and  West  Coasts. — 
In  middle  and  higher  latitudes  (beyond  300  or  400),  a 
marked  difference  of  climate  exists  in  corresponding  lat- 
itudes between  the  east  and  west  coasts  of  the  continents. 
This  is  caused  chiefly  by  the  relation  between  the  conti- 
nents and  the  direction  of  the  winds.  In  these  latitudes 
the  general  movement  of  the  (antitrade)  winds  is  from 
the  west.  In  winter,  however,  on  account  of  the  dif- 
ference of  temperature  between  the  land  and  sea  air,  a 
great  anticyclone  tends  to  form  over  the  cold  interior  of 
the  continents,  and  a  great  cyclone  over  the  warmer 
oceans.  The  course  of  the  air  in  passing  out  of  the  anti- 
cyclone into  the  cyclone  is  such,  in  the  northern  hemi- 
sphere, as  to  make  winds  from  the  north-west  prevalent  on 
the  east  side  of  continents,  and  from  the  south-east  or 
south  on  the  west  side  at  this  season  (see  Wind  Chart, 
page  87,  January).  Hence,  the  eastern  winters  are  much 
colder  than  the  western,  since  the  east  side  is  flooded  with 
dry  air  from  the  intensely  cold  northern  part  of  the 
interior,  while  the  west  side  is  covered  with  air  from  the 
relatively  warm  southern  part.  Furthermore,  the  winters 
on  the  east  side  are  relatively  dry,  since  the  air  is  advancing 
into  lower  latitudes  and  hence  becoming  warmer.  On  the 
west  side,  however,  the  winters  are  relatively  moist,  since 
the  air  is  advancing  into  colder  latitudes  and  constantly 
increasing  in  relative  humidity.  In  summer,  on  the  con- 
trary, the  relatively  warmer  air  over  the  land  tends  to  form 
a  cyclone  over  the  interior  of  the  continents  into  which 
the  surrounding  air  whirls,  resulting  in  southerly  winds 
on  the  east  side,  and  northerly  winds  on  the  west  side  of 
the  continents  (see  Wind  Chart,  page  87,  July).  Hence, 
the  summers  on  the  east  side  are  relatively  warm  and 
moist,  while  those  on  the  west  side  are  relatively  cool  and 
dry.     In  general,  the  east  side  of  continents  in  middle  and 


WEATHER  AND    CLIMATE. 


303 


higher  latitudes  has  a  continental  climate  with  abnormally 
low  mean  temperature,  and  the  greatest  rain-fall  in  sum- 
mer; while  the  west  side  has  a  moderately  oceanic  climate 
with  abnormally  high  mean  temperature,  and  the  greatest 
rain-fall  in  winter. 


UNITED  STATES  43 >20LAT.                                   EURO-ASIA    52°  LAT. 

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Fig.  121. 


On  the  diagram  (Fig.  121),  this  is  well  shown  for  middle  latitudes 
in  both  North  America  and  Euro-Asia.  It  is  seen  that  while  on  the 
east  coasts  the  range  is  not  quite  so  great  as  in  the  interior,  it  is 
much  greater  than  on  the  west  coast,  the  summers  being  slightly- 
warmer  and  the  winters  much  colder  ;  hence,  the  mean  temperature 
is  lower.  The  range  is  greater  in  Euro-Asia  than  in  North  America 
because  the  grand  division  is  larger  and  the  seasonal  winds  stronger. 
The  rain-fall  curves  show  that  on  the  east  side  of  both  grand  divi- 
sions most  of  the  precipitation  occurs  in  summer,  while  on  the  west 
side  most  of  it  occurs  in  winter.  In  the  United  States,  the  Rocky 
Mountains  and  the  Colorado  River  roughly  divide  the  east  from  the 
west  side  in  the  matter  of  winter  and  summer  rain-fall.  In  latitude 
6o°  N.,  the  east  coast  of  each  grand  division  is  about  200  colder  than 
the  west  coast,  and  has  a  range  about  3  50  greater.    These  differences 


3O4  PHYSICAL    GEOGRAPHY. 

decrease  southwardly  and  disappear  within  the  tropics.  This  partly 
explains  why  the  east  coast  of  North  America  in  middle  and  higher 
latitudes  has  a  colder  and  more  extreme  climate  than  the  opposite 
European  shores  in  corresponding  latitudes ;  thus,  New  York  City 
has  a  mean  temperature  8°  less,  and  a  range  260  greater,  than  the 
opposite  coast  of  Portugal.  In  the  same  way,  the  climate  of  our 
north-west  Pacific  coast  is  more  moderate  than  that  of  the  opposite 
Asiatic  coast.  For  the  same  general  reason  (direction  of  prevailing 
winds),  there  is  a  similar  though  small  climatic  difference  between 
the  east  and  west  coasts  of  peninsulas  or  islands,  or  of  great  inland 
lakes.  Thus,  Milwaukee,  on  the  west  shore  of  Lake  Michigan,  has 
a  mean  temperature  2°  lower,  and  a  range  4)4°  greater, than  Grand 
Haven,  Mich.,  in  the  same  latitude  and  only  80  miles  distant,  but  on 
the  east  shore  of  the  lake.  The  precipitation  is  16%  greater  at 
Grand  Haven,  and  is  greatest  in  autumn  rather  than  in  summer,  as 
at  Milwaukee.    • 

Ocean  currents  come  from  warmer  or  colder  regions, 
and  hence  bring  water  abnormally  warm  or  cold  for  the 
latitude;  and  this  modifies  the  temperature  of  the  over- 
lying air.  If  this  air  is  brought  by  wind  to  the  land, 
then  one  may  say  that  currents  influence  the  climate  of 
adjacent  coasts.  The  currents  on  the  west  side  of  all 
oceans  move  from  the  equator  in  tropical  latitudes  and  are 
relatively  warm,  while  on  the  east  side  of  the  tropical 
oceans  currents  move  toward  the  equator,  and  are  rela- 
tively cool.  Hence,  the  east  coasts  of  the  continents  in 
equatorial  regions  have  a  warmer  climate  than  west 
coasts.  The  reverse  is  the  case  in  the  higher  latitudes, 
where  cold  currents  move  toward  the  equator  in  the 
western  part  of  the  oceans,  and  warm  currents  move 
from  the  equator  in  the  eastern  part  (page  137). 

Thus,  the  west  coast  of  Africa,  and  the  west  coast  of  America 
from  400  N.  to  400  S.,are  abnormally  cool,  while  the  east  coasts  of 
America  from  South  Carolina  to  Cape  Horn,  and  the  whole  east 
coasts  of  Africa  and  Australia  have  abnormally  warm  climates, 
owing  to  adjacent  ocean  currents.  It  is  only  opposite  the  equatorial 
calms,  in  which  the  counter-current  carries  warm  water  eastward, 


(305) 


306  PHYSICAL    GEOGRAPHY. 

that  inter-tropical  west  coasts  are  not  relatively  cool.  In  higher 
latitudes  (above  400  N.  and  S.)  the  warm  currents  wash  the  west 
coasts,  and  aid  the  west  winds  slightly  in  producing  a  moderate 
climate,  while  the  cold  currents  adjacent  to  the  opposite  east  coasts 
exercise  an  equal  influence  in  depressing  the  climatic  temperature. 

The  amount  of  precipitation  in  coast  regions  also 
depends  conjointly  upon  the  direction  of  the  winds  and 
neighboring  ocean  currents.  Warm  currents — those  flow- 
ing from  lower  latitudes-  -tend  to  produce  a  large  precipi- 
tation, since  the  air  over  them  is  nearly  saturated  and  is 
abnormally  warm.  It  is  therefore  cooled  and  part  of  its 
vapor  condenses,  when  transferred  in  any  direction  from 
over  the  current.  Cold  currents  tend  to  prevent  precipi- 
tation for  contrary  reasons. 

The  general  distribution  of  rain-fall  (see  opposite  chart  and  one 
on  page  76),  shows  the  connection  between  atmospheric  precipita- 
tion, the  temperature  of  the  ocean  currents,  and  the  direction  of 
the  winds.  It  is  seen  that  in  the  lower  latitudes,  to  about  the  lat- 
itude of  Cape  Mendocino,  Norfolk,  Gibraltar,  and  Japan  on  the 
north,  and  Rio  de  la  Plata,  Cape  of  Good  Hope,  and  south  Aus- 
tralia on  the  south,  the  east  sides  of  the  continents  enjoy  east  winds, 
are  washed  by  abnormally  warm  currents,  and  receive  the  heaviest 
rain -fall.  In  higher  latitudes,  embracing  the  northern  parts  of 
North  America  and  Euro -Asia,  and  the  southern  part  of  South 
America  and  Tasmania,  we  have  west  winds  with  warm  currents 
off  the  west  coasts,  and  the  west  sides  of  continents  in  these  lat- 
itudes receive  the  heavy  rain-fall. 

The  only  places  in  tropical  latitudes  where  heavy  rain-fall  occurs 
on  west  coasts  are  close  to  the  equator,  where  the  equatorial  counter- 
current  brings  warm  water  against  these  coasts,  and  in  India  and 
Farther  India,  where  the  seasonal  winds  are  very  strong,  and  the 
configuration  of  these  mountainous  west  coasts  is  peculiarly  adapted 
to  cool  the  moist  south-west  monsoon  of  summer. 

In  the  torrid  zone,  where  the  great  uniformity  of 
temperature  prevents  a  thermal  division,  the  year  .is  divided 
by  the  variation  in  the  amount  of  rain-fall  into  a  wet  season 
and  a  dry  season.     Local  peculiarities  largely  determine 


?00°3   m  "HZ 
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(307) 


308  PHYSICAL    GEOGRAPHY. 

the  time  of  occurrence  of  these  seasons  at  different  places, 
but  in  general  the  wet  or  rainy  season  occurs  when  the 
thermal  equator  crosses  or  lies  in  the  vicinity  of  any  lo- 
cality; and  this  follows  more  or  less  closely  the  passage 
of  the  sun  through  the  zenith  of  the  locality.  Near  the 
thermal  equator  the  motion  of  the  air  is  upward ;  and  as  it 
cools  in  rising,  its  vapor  condenses  into  clouds  arid  rain. 

Since  the  sun  is  twice  annually  in  the  zenith  of  all  places  be- 
tween the  tropics,  there  is  a  tendency  toward  two  rainy  and  two 
dry  seasons  each  year  in  the  torrid  zone ;  but,  excepting  near  the 
equator  where  nearly  six  months  intervene  between  the  passages 
of  the  sun  through  the  zenith,  the  two  rainy  seasons  merge  into, 
one,  thus  dividing  the  year  into  one  moderately  long  wet  season 
and  one  very  long  dry  season. 

Influence  of  Elevation  on  Climate, — The  climate  of 
highlands  every-where  has  certain  general  peculiarities 
which  distinguish  it  from  that  of  adjacent  lowlands. 
Prominent  among  these  are  a  lower  mean  annual  tempera- 
ture, and  a  greater  difference  between  the  temperature  of  the 
air  and  that  of  the  ground  or  surface  objects. 

The  air  resting  on  highlands  is  less  dense,  is  clearer,  and  con- 
tains less  vapor  than  that  resting  on  lowlands,  and  hence  has  fewer 
molecules  to  absorb  the  heat  of  the  entering  sunbeams  by  day  or 
of  the  outward -passing  earth  radiations  at  night.  Therefore,  the 
highland  air  must  in  general  be  cooler  than  that  resting  on  low- 
lands. This  is  well  shown  by  the  distribution  of  mean  annual  tem- 
perature of  surface  air  in  the  United  States,  where  the  peculiar 
southward  extension  of  the  isotherm  of  500  in  the  east  and  of  400 
in  the  west  is  caused  by  the  highlands  of  the  Appalachian  and 
Rocky  mountains  respectively.  But  for  the  very  reason  that  the 
rays  lose  less  of  their  heat  in  passing  through  highland  air,  the 
arrival  or  departure  of  these  rays  produces  a  greater  heating  or 
cooling  effect  upon  the  ground  and  surface  objects  than  in  lowlands  ; 
that  is,  the  ground  on  the  highlands,  when  exposed  to  the  sun  (by 
day  or  in  summer),  becomes  hotter  than  the  overlying  air  or  than 
lowland  ground.  On  the  other  hand,  when  not  directly  exposed  to 
the  sun  (at  night  or  in  winter),  the  highland  ground  may  become 


(309) 


3IO  PHYSICAL   GEOGRAPHY. 

colder  than  either  the  overlying  air  or  the  lowland  ground  (p.  21). 
Vegetation  fails  on  high  mountains  (even  near  the  equator),  not 
because  the  sun  does  not  supply  sufficient  heat,  but  because  the 
evaporation  is  too  great,  and  the  rare,  dry  air  can  not  retain  the 
heat  near  the  earth's  surface,  and  thus  allow  it  to  accumulate  from 
day  to  day.  In  the  lowlands  of  polar  regions,  on  the  contrary, 
vegetation  does  not  thrive,  partly  because  the  sun's  rays  fall  so 
obliquely  that,  though  the  dense  lower  air  permits  the  heat  to  accu- 
mulate during  several  months  of  constant  day,  the  aggregate  is 
only  sufficient  to  support  a  stunted  vegetable  life. 

The  exposure,  or  direction  of  slope,  in  hilly  country 
has  a  great  influence  on  the  amount  of  heat  imparted  to 
the  ground,  and  hence  upon  the  climate.  In  the  northern 
hemisphere  the  southern  slopes  receive  the  rays  more  per- 
pendicularly, and  for  a  longer  time  each  day,  and  are 
hence  warmer  than  the  other  slopes.  In  the  southern 
hemisphere  the  northern  slopes  are  the  warmest.  The 
higher  temperature  of  the  ground  affects  the  overlying  air, 
and  makes  the  climate  more  moderate.  Therefore,  other 
things  being  equal,  the  lower  limit  of  perpetual  snow, 
and  the  higher  limit  of  vegetation,  lie  at  a  greater  height 
on  the  south  than  on  the  north  slopes  of  mountains  in 
our  hemisphere. 

The  rate  at  which  the  air  becomes  cooler  as  the 
observer  ascends,  varies  at  different  places,  and  at  different 
times  at  the  same  place.  The  general  average  is  about 
i°  Fahr.  for  each  300  to  350  feet  of  elevation  on  a  slope 
whose  acclivity  is  quite  steep,  as  on  a  mountain  side,  but 
it  is  sometimes  as  rapid  as  i°  for  every  200  feet,  and 
sometimes  as  slow  as  i°  for  every  500  feet.  The  rate  is 
generally  most  rapid  in  summer,  and  on  the  warm  side  of 
a  mountain.  The  rate  is  much  slower  on  gentle  acclivities 
than  upon  steep  slopes.  On  the  ordinary  slopes  of  non- 
mountainous  regions,  as  the  great  Mississippi  Valley,  the 
average  rate  is  about   i°  for  each  450  feet  of  ascent. 


WEATHER  AND    CLIMATE.  311 

Peculiarities  of  vertical  distribution  of  temperature 
in  hilly  regions.  During  calm,  clear  nights,  especially  in 
winter,  in  middle  and  higher  latitudes,  it  is  observed  that 
up  to  a  certain  height  the  air  in  valleys  is  colder  than  that 
on  the  slopes  of  surrounding  eminences.  Over  open 
plains  it  has  also  been  observed  that  the  temperature  dur- 
ing calm,  clear  nights  increases  with  elevation.  This  in- 
crease of  temperature  extends  at  least  to  a  height  of  150 
feet,  and  is  most  rapid  in  the  lowest  layers  of  air,  where 
it  may  attain  a  rate  of  i°  in  5  feet,  or  even  more.  Thus, 
on  frosty  nights  the  tree-tops  frequently  remain  unharmed, 
while  the  lower  foliage  and  herbage  are  frozen. 

The  earth  cools  quickly  on  clear  nights  by  radiating  its  heat 
through  the  overlying  air.  The  air  cools  much  less  except  where 
dusty  or  damp  enough  to  have  effective  radiating  power.  Thus,  the 
lower  air  is  chilled  by  contact  with  the  colder  earth.  On  valley 
slopes  the  cooled  and  hence  heavy  surface  air  creeps  down  to  the 
lowest  ground,  where  it  accumulates,  lifting  up  the  relatively  warm 
air  that  it  finds  there.  Accordingly,  there  is  a  climatic  tendency 
toward  warmer  nights  on  slopes  and  hill-tops  than  in  adjacent  val- 
leys. In  latitudes  where  frosts,  though  infrequent,  sometimes  occur, 
this  peculiarity  is  of  great  importance  to  the  agriculturist,  since  the 
frosts,  though  occurring  in  the  valleys,  may  never  occur  on  the 
higher  grounds.  A  region  calle.d  the  Thermal  Belt,  in  the  Appa- 
lachian Range,  is  thus  specially  favored. 

Mountains  tend  to  produce  condensation  of  atmos- 
pheric vapor  in  all  parts  of  the  world,  since  the  lower  and 
moister  air-currents  are  compelled  to  ascend  in  crossing  a 
mountain  range,  and  are  thus  cooled  by  expansion. 
Hence,  mountain  slopes  to  a  certain  height  usually  have  a 
moister  climate,  that  is,  they  have  more  clouds  and  rain, 
than  the  surrounding  lowlands.  Thus,  in  the  plateau  re- 
gion of  the  West,  many  of  the  mountain  ranges  and  higher 
mesas  have  a  sufficient  rain-fall  to  support  quite  a  heavy 
growth  of  forest,   while  on  the  lower  general  surface  of 


312  PHYSICAL    GEOGRAPHY. 

the  country,  the  rain-fall  is  so  slight  that  prairie  grass, 
sage-brush,  and  cactus  are  the  only  forms  of  vegetation, 
except  along  the  streams  that  carry  off  the  surplus  rain- 
fall of  the  mountains.  Even  in  the  center  of  the  intensely 
dry  desert  of  Sahara,  the  higher  mountain  regions  of 
Asben  and  Tibesti  have  a  regular  summer  rain-fall. 

Mountain  ranges  have  a  moist  side  and  a  dry  side 
when  they  trend  more  or  less  directly  across  the  direction 
of  the  prevailing  winds.  In  the  torrid  zone,  where 
easterly  winds  prevail,  the  east  slope  is  usually  the  moist 
side,  —  as,  for  instance,  the  American  Cordilleras  from 
Mexico  to  northern  Chile.  In  higher  latitudes  the  west 
side  of  mountain  ranges  usually  receives  the  greatest  rain- 
fall,— as,  for  examples,  the  Cascade  Range  in  Oregon  and 
Washington  and  the  Andes  of  southern  Chile.  Mountains 
whose  trend  is  nearly  parallel  with  the  course  of  the  wind, 
as  the  Appalachians  and  the  Alps,  have  no  well  marked 
wet  and  dry  sides. 

In  crossing  a  mountain  range,  the  air  loses  by  condensation  on 
the  windward  slopes  all  the  vapor  it  contains  in  excess  of  the 
amount  which  saturates  it  at  the  lowest  temperature  it  attains  when 
near  the  crest.  In  gradually  sinking  on  the  further,  or  lee,  side  of  the 
mountain,  the  air  is  mechanically  warmed,  and  hence  its  relative 
humidity  decreases.  This  not  only  produces  an  excessively  dry 
climate,  but  operates,  also,  to  raise  the  mean, and  increase  the  range 
of  temperature  on  the  lee  side  of  the  mountain,  for  the  dry  air  and 
cloudless  sky  favor  intense  heating  of  the  earth's  surface  by  day, 
and  rapid  cooling  by  radiation  at  night,  while  on  the  windward  side 
the  rising  air  favors  the  formation  of  clouds  and  mists,  which  pre- 
vent intense  heating  of  the  earth  by  day,  or  extreme  cooling  by 
night. 


PART   VI.— LIFE. 
CHAPTER  XXII. 

THE   VARIOUS    FORMS    OF    LIFE. 

My  substance  was  not  hid  from  thee,  when  I  was  made  in  secret,  and  curiously 
wrought  in  the  lowest  parts  of  the  earth.  Thine  eyes  did  see  my  substance,  yet  be- 
ing unperfect ;  and  in  thy  book  all  my  members  were  written,  which  in  continuance 
were  fashioned,  when  as  yet  there  was  none  of  them. — Psalm  cxxxix:  15,  16. 

Life  is  a  mysterious  and  temporary  manifestation  in  a 
peculiar  kind  of  matter.  This  kind  of  matter  is  called 
protoplasm.  The  chemical  composition  of  this  substance 
is  very  imperfectly  understood,  but  it  is  known  to  consist 
chiefly  of  carbon,  oxygen,  hydrogen,  nitrogen,  and  sul- 
phur, in  various  combinations,  which  differ  somewhat  in 
different  kinds  of  protoplasm.  But  in  all  kinds,  certain 
highly  complex  compounds  of  these  substances,  called 
proteidsy  are  practically  identical,  and  only  matter  in  which 
these  proteids  are  present  is  known  to  manifest  the  prop- 
erties of  life. 

All  matter  in  the  living  state  is  closely  associated  with  lifeless 
matter  in  the  same  body  or  structure;  thus,  the  fat,  parts  of  the 
hair,  nails,  and  blood,  most  of  the  horns  or  shells  of  living  animals, 
and  the  bark,  solid  wood,  and  sap  of  living  plants,  are  composed 
of  matter  in  a  perfectly  lifeless  condition.  Science  has  never  dis- 
covered what  causes  this  wonderful  difference  of  condition  in  mat- 
ter, but  so  far  as  we  know,  the  living  state  is  never  assumed  except 
under  the  influence  of  existing  living  matter,  which  seems  to  infect 
lifeless  protoplasm,  and  in  some  way  causes  it  to  assume  the  living 

state. 

(313) 


314  PHYSICAL    GEOGRAPHY. 

Living  matter  is  distinguished   from  lifeless  matter 

(1)  by  its  power  of  repairing  its  waste,  and  of  growth,  and 

(2)  by  its  power  of  reproduction.  While  a  mass  of  mat- 
ter is  in  the  living  state,  portions  of  it  are  constantly 
dying  and  being  thrown  off,  but  the  living  portion  contin- 
ually repairs  the  loss  by  a  process  called  intussusception. 
This  consists  in  manufacturing  appropriate  kinds  of  new 
particles,  and  fitting  them  into  the  interstices  between 
those  present,  throughout  the  whole  mass.  If  this  proc- 
ess exceeds  the  loss,  the  living  mass  increases  in  size,  or 
grows.  In  this  respect,  living  matter  differs  widely  from 
lifeless  matter,  which  grows,  if  at  all,  only  by  the  addi- 
tion of  particles  to  its  surfaces.  Living  matter  not  only 
repairs  its  waste,  and  grows,  but  under  certain  circum- 
stances detaches  from  itself  masses  of  living  matter  which 
are  endowed  with  all  the  properties  of  growth  and  repro- 
duction possessed  by  the  parent  mass. 

Organisms. — Living  bodies  of  all  but  the  lowest  forms 
are  composed  of  unlike  parts,  each  capable  of  performing 
different  functions  essential  to  the  life  of  the  whole  body. 
These  unlike  parts,  such  as  the  stomach,  heart,  limbs, 
etc.,  in  animals,  and  roots,  stem,  leaves,  etc.,  in  plants, 
are  called  organs,  and  the  whole  body  is  called  an  organ- 
ism because  it  possesses  them ;  while  lifeless  protoplasm 
is  frequently  called  organic  matter,  because,  so  far  as 
known,  it  has  invariably  been  produced  in  living  bodies. 
Cells. — All  organisms  exist  at  first  as  a  minute  mass 
of  protoplasm  called  the  germ-cell  (Fig.  122), 
forming  part  of  the  body  of  the  parent,  from 
which  it  becomes  detached  when  the  new 
organism  has  reached  the  proper  stage  of  its 
Fig.  iaa.  development.  The  protoplasm  of  the  germ- 
cell  (p)  is  a  transparent,  jelly-like  mass  resembling  white 
of  Qgg,  and  part  of  it  is  usually  gathered  into  a  darker, 


THE    VARIOUS    FORMS    OF    LIFE. 


315 


rounded  nucleus  (n),  while  the  whole  may  or  may  not  be  en- 
veloped in  a  membrane  or  sack  of  soft,  lifeless  material 
forming  the  cell-wall  (m).  When  sufficiently  magnified,  the 
living  protoplasm  is  seen  to  be  always  in  motion,  regular 
currents  traversing  its  mass  in  more  or  less  definite  direc- 
tions. 

The  simplest  forms  of  life  are  organisms  similar  to 
the  germ-cell,  leading  an  independent  existence,  and  re- 
producing similar  forms  by  simply  dividing  into  two  or 

Fig.  123.— Various  Stages  in  the  life  of 


more  similar  masses  of  protoplasm.  Such  simple  yet 
complete  organisms  are  the  Protococcus  and  common  Yeast 
plants,  and  the  Amoeba  animalcule. 

In  all  higher  forms  of  life,  the  germ-cell  develops 
by  subdivision,  or  segmentation,  through  two,  four,  eight, 
sixteen,  etc.,  into  a  great  number  of  nucleated  cells  of 
protoplasm  within  the  original  cell-wall,  which  finally  dis- 
appears. To  this  point  the  new  cells  closely  resemble 
each  other,  being  all  nearly  spherical,  or  varying  from 
mat  form  only  by  their  pressure  against  one  another.  But 
p  G.-18. 


316 


PHYSICAL    GEOGRAPHY. 


Fig.  124.— Segmentation  of  a  Cell. 


the  further  development  of  the  organism  is  still  more 
wonderful.  New  cells  continue  to  be  formed  by  seg- 
mentation,  but  the  cells  in  different   parts  of  the  mass 

begin     to    surround    them- 


selves with  cell-walls  of  life- 
less matter,  and  to  adapt 
themselves  for  the  various 
kinds  of  work  they  have  to 
do,  by  gradual  differentiation; 
that  is,  assuming  different 
shapes  and  structures.  In 
many  parts  of  the  organism 
the  protoplasm  may  nearly  or  entirely  disappear  in  the 
production  of  cell-walls,  thus  producing  a  lifeless  but  solid 
or  cellular  portion  of  the  organism,  as  the  woody  part  of 
plants,  and  the  bones  and  outer  skin  of  animals.  By  con- 
tinued segmentation  and  differentiation  of  the  originally 
similar  cells,  the  very  dissimilar  organs  of  the  organism 
are  finally  formed,  and  each  organ,  when  fully  developed, 
is  thus  entirely  composed  of  variously  modified  cells  of 
living  protoplasm,  separated  by  more  or  less  cellular  walls 
of  lifeless  matter  of  various  thickness,  shape,  and  sub- 
stance. 

Respiration. — The  development  and  growth  of  every 
organism  as  a  whole  is  thus  the  result  of  the  death  and 
destruction  of  portions  of  its  protoplasm.  The  general 
process  by  which  this  destruction  is  accomplished  is  the 
same  in  all  organisms ;  they  all  exhibit  the  phenomenon 
of  respiration,  or  breathing.  Land  organisms  inhale  atmos- 
pheric oxygen  directly,  while  aquatic  organisms  inhale  that 
which  is  dissolved  in  the  water.  The  strong  chemical 
affinity  of  oxygen  for  all  other  elements  enables  it  to  de- 
compose the  complex  protoplasmic  substances  of  the 
organism  and  form  simpler  and  more  stable  compounds, 


THE   VARIOUS   FORMS   OF    LIFE.  317 

organic  energy  and  heat  being  liberated  by  the  change. 
One  of  the  stable  compounds  is  carbonic  acid,  which  is 
largely  expired,  or  breathed  out,  by  all  organisms.  Thus, 
respiration  is  directly  a  destructive  process,  since  it  results 
in  the  killing  and  removing  of  portions  of  the  organism. 
Animal  and  Vegetable  Kingdoms. — The  material 
with  which  the  loss,  occasioned  by  respiration,  is  continu- 
ally repaired,  is  manufactured  from  the  food  of  the  organ- 
ism, the  process  being  called  nutrition;  and  since  it 
consists  of  the  conversion  of  lifeless  food  into  living  pro- 
toplasm, it  is  a  constructive  process,  and  therefore  directly 
opposed  to  respiration.  It  is  in  connection  with  nutrition 
that  the  essential  difference  between  plants  and  animals 
occurs.  Since  all  living  protoplasm  contains  the  proteid 
combinations  of  carbon,  oxygen,  hydrogen,  nitrogen,  etc., 
the  food  of  all  organisms  must  contain  these  substances; 
but  plants  alone  are  able  to  manufacture  the  complex  pro- 
teids  out  of  simpler  and  more  stable  combinations  of 
these  elements,  while  animals  require  food  in  which  the 
proteids  exist  already  manufactured.  Hence,  the  animal 
kingdom  depends  absolutely  upon  the  vegetable  kingdom 
for  its  food. 

All  green  plants,  which  form  by  far  the  larger  portion  of  the 
vegetable  kingdom,  can  manufacture  their  food  only  in  the  sunlight 
(direct  or  diffused),  and  these  plants  obtain  their  food  chiefly  from 
two  sources :  carbon  they  obtain  mostly  from  the  carbonic  acid  in 
the  air,  through  minute  mouths  (stomata)  in  the  under  side  of  the 
leaf;  hydrogen  and  oxygen  are  derived  chiefly  from  the  water  ab- 
sorbed by  the  roots,  though  plants,  like  animals,  also  obtain  oxygen 
by  respiration  from  the  atmosphere  ;  nitrogen,  sulphur,  and  other 
elements  are  derived  chiefly  as  various  salts  dissolved  in  the  water. 
By  the  aid  of  the  kinetic  energy  in  the  sunlight,  these  green  plants 
are  enabled  to  decompose  the  water  and  carbonic  acid,  returning  to 
the  atmosphere  part  of  the  oxygen  thus  disengaged,  but  uniting  the 
hydrogen  and  carbon  with  the  rest  of  the  oxygen  to  form  a  carbo- 
hydrate— starch.     Sooner  or  later  this  is  changed  into  a  kind  of 


31  8  PHYSICAL    GEOGRAPHY. 

sugar  (glucose),  and,  dissolved  in  the  sap,  is  transferred  to  the  point 
where  new  protoplasm  is  needed.  Here,  in  some  unknown  way,  it 
unites  with  the  nitrogen  and  sulphur  to  form  a  proteid,  and  the 
newly  made  protoplasm  becomes  endowed  with  the  properties  of 
life.  A  few  plants,  as  the  fungi,  bacteria,  and  common  yeast-plant, 
do  not  require  sunlight,  but  can  live  in  darkness.  These  plants, 
like  animals,  require  organic  food ;  but,  unlike  animals,  can  manu- 
facture proteids,  if  only  a  carbohydrate  is  present  in  their  food.  In 
this  respect  these  organisms  occupy  an  intermediate  position  be- 
tween the  animal  and  vegetable  kingdoms.  In  almost  all  animals 
the  region  where  nutrition  occurs  is  completely  and  more  or  less 
directly  inclosed  by  layers  of  cell-walls  or  membranes,  but  the  pro- 
teids in  food  are  indiffusible  ;  that  is,  unable  to  pass  through  a  mem- 
brane. Hence,  the  food  has  to  be  made  diffusible  before  it  can 
enter  the  system.  It  is  to  effect  this  preparatory  change  in  the 
food,  called  digestion,  that  animals  require  a  stomach,  which  is  es- 
sentially a  more  or  less  complicated  pouch  formed  by  the  infolding 
of  the  outer  surface  of  the  body.  Thus,  it  is  only  after  the  digested 
food  has  passed  through  the  walls  of  the  stomach  that  it  really 
enters  the  body.  Plants,  which  manufacture  the  proteids  within 
themselves,  and  some  of  the  lowest  animals,  which,  being  minute 
naked  masses  of  protoplasm,  can  receive  their  food  by  simply  flow- 
ing over  and  enveloping  it,  of  course  require  no  stomach. 

Two  great  laws  of  the  organic  world  have  been  es- 
tablished from  prolonged  observation  of  living  things:  1st, 
The  Law  of  Heredity ;  organisms  reproduce  others,  which  at 
maturity  closely  resemble  their  parents.  Though  the  resem- 
blance is  close,  the  likeness  is  never  exact,  and  this  leads, 
2d,  to  the  Law  of  Adaptation;  all  organisms  possess,  in 
greater  or  less  degree,  the  power  to  adapt  themselves  to 
gradual  changes  in  their  surroundings,  or  environment. 

It  is  a  well  known  fact  that  family  resemblances  may  generally 
be  traced  from  one  generation  to  another,  but  no  two  human  beings 
are  so  exactly  alike  in  all  particulars  that  intimate  friends  can  not 
distinguish  certain  differences.  The  same  is  true  of  all  animals  and 
plants :  there  is  a  close  resemblance  running  through  the  various 
families,  but  no  two  organisms  are  exactly  alike,  though  people  gen- 
erally are  not  sufficiently  well  acquainted  with  them  to  recognize  at 
once  individual  peculiarities.     The  power  of  adaptation  is  illustrated 


THE   VARIOUS    FORMS    OF    LIFE.  319 

not  only  by  the  alteration  in  the  appearance  of  plants  and  in  the 
quality  of  the  fur  of  many  animals  as  the  seasons  change,  but  by 
the  alteration  in  the  skin  and  muscles  of  men  and  women  which 
follows  certain  changes  in  their  mode  of  living,  as  from  an  indoor, 
inactive  life,  to  one  of  hard  manual  labor  and  exposure  to  the  sun 
and  elements. 

The  environment  of  an  organism  embraces  every 
thing  outside  of  itself  that  affects  in  any  way  the  condi- 
tions of  its  existence.  It  embraces  (1)  all  the  factors  that 
influence  the  food  supply  of  the  organism ;  (2)  all  the 
factors  of  climate ;  (3)  all  the  factors  that  determine  the 
presence  or  absence  of  other  plants  or  animals  that  inter- 
fere with  or  promote  the  well-being  of  the  organism ;  and 
(4)  every  thing  that  modifies  any  one  of  these  factors.  It 
is  inconceivable  that  all  these  factors  can  ever  be  exactly 
alike  at  two  different  localities  or  at  two  different  times. 
Hence,  every  organism  has  a  different  environment  which 
is  constantly  changing  to  a  greater  or  less  extent,  and  the 
constant  adaptation  of  an  organism  to  its  special  environ- 
ment probably  accounts  for  its  individual  peculiarities. 

Classification. — The  grouping  of  organisms  according 
to  the  degree  of  similarity  in  structure  or  function  of  their 
corresponding  parts,  constitutes  classification.  The  first 
and  broadest  grouping  of  living  things  is  into  the  vegeta- 
ble and  animal  kingdoms.  Each  kingdom  is  then  divided 
into  several  smaller  groups,  and  these  into  others,  which 
in  turn  are  subdivided  again  and  again.  Each  of  the  two 
largest  divisions  embraces  organisms  which  are  widely  dis- 
similar in  almost  every  respect  excepting  mode  of  nutrition, 
while  each  of  the  successively  smaller  groups  is  character- 
ized by  a  greater  and  greater  number  of  similarities  be- 
tween the  organisms  of  which  it  is  composed,  until,  in  the 
smallest  groups,  of  which  there  may  be  a  million  or  more, 
all  the  individual  organisms  of  each  group,  while  not 
exactly  alike,  resemble  each  other  so  closely  in  structure 


320 


PHYSICAL    GEOGRAPHY. 


and  function  that  they  are  said  to  constitute  a  single  kind, 
or  species,  of  plants  or  animals.  Thus,  there  are  a  number 
of  varieties  of  apples,  and  yet  they  are  all  sufficiently 
similar  to  be  classed  as  a  single  species  of  the  vegetable 
kingdom;  and  in  the  same  way  all  chickens,  though  no 
two  are  exactly  alike,  are  essentially  similar,  and  are 
classed  as  a  single  species  of  the  animal  kingdom. 

The  characteristic  similarities  which  determine  some  of  the  larger 
groupings  and  subgroupings  of  the  organic  world  are  given  below, 
the  groups  embracing  the  simplest  or  lowest  forms  of  vegetable  and 
animal  life  respectively  being  placed  first,  and  those  containing  the 
most  complex  or  highly  organized  forms  being  placed  last. 

Vegetable  Kingdom, 


Fig.  125. 


Fig.  126. 


Fig.  127. 


all  organisms  able  to  manufacture  the  complex  proteids. 

a.  Cryptogamia  {hidden  seeds).  All  flowerless 
plants.     Subdivided  into : 

1.  Protophytes  {first plants').  Simplest  and  lowest 
plants.  Generally  microscopic ;  either  single  cells  or 
an  association  of  cells  without  mutual  dependence,  as 
the  diatoms,  moulds,  bacteria,  yeast,  etc.  (Fig.  123). 

2.  Thallogens  {shoot  growers).  Many  cells,  but 
without  differentiation  into  stem  and  leaf;  growing 
horizontally  in  spreading  shoots  or  fronds,  as  the 
alga,  or  sea-weeds ;  fungi,  or  toad-stools  (Fig.  125); 
and  the  lichens. 

3.  Bryogens  {moss  growers).  Cells  differentiated 
into  root,  stem,  and  leaf,  but  no  woody  material; 
showing  tendency  to  grow  upward  rather  than  hori- 
zontally, as  the  liverworts  and  mosses  (Fig.  126). 

4.  Acrogens  {highest growers).  Cells  differentiated 
more  completely,  the  stem  and  leaves  containing 
vascular,  woody  fibers ;  showing  strong  tendency  to 
grow  upward,  as  the  ferns  (Figs.  127,  129). 

b.  Phenogamia  {visible  seeds).  All  flowering  plants. 
Subdivided  into : 

1.  Gymnosperms  {naked  seeds).  Flowering  plants 
which  do  not  inclose  their  seeds  in  seed-vessels.    The 


THE    VARIOUS    FORMS    OF    LIFE. 


321 


group  is  subdivided  into  the  (a)  cycads  (palm  ferns,  Fig.  128),  (&) 
conifers  (pines,  firs,  spruces,  larches,  cypresses,  cedars,  etc.),  and 
{c)  gnetums. 


2.  Angiosperms  {seed-vessels). 
their  seeds  in  seed-vessels.  The 
group  is  subdivided  into  (a) 
monocotyledons  (single  lobed), 
which  first  develop  a  single  seed 
leaf,  or  lobe,  and  are  character- 
ized by  leaves  having  parallel 
veins  ;  by  three-petaled  flowers  ; 
and  by  the  absence  of  a  distinct 
pith  and  lines  of  annual  growth 


Flowering  plants  which  inclose 


Fig. 128. 


Fig.  129. 


in  the  stem,  as  the  rushes,  grasses,  (cereals,  corn,  cane,  etc.),  lilies, 
bananas,  and  true  palms  (Fig.  130) ;  {b)  dicotyledons  (double  lobed), 
which  first  develop  a  pair  or  more  of  seed  leaves,  or  lobes,  and  are 
characterized  by  leaves  having  netted  veins ;  and  by  the  division  of 
the  stem  into  a  central  pith,  an  outside  bark, 
and  a  series  of  concentric  layers  of  wood  be- 
tween them,  an  additional  layer  of  wood  being 
added  beneath  the  bark  by  each  season's 
growth.  This  group  includes  most  garden 
vegetables,  fruit-trees,  and  hard -wood  forest 
trees.  It  is  subdivided  into :  (a)  monochlamyds 
(single  cloaks),  or  plants  whose  flowers  consist 
of  but  a  single  whorl  of  leaves  (the  calyx),  em- 
bracing the  catkin-bearing  plants,  as  willows, 
poplars,  beeches,  oaks,  elms,  laurels,  hemp, 
hops,  etc.;  and  (b)  dichlamyds  (double  cloaks) 
or  plants  whose  flowers  consist  of  a  double 
whorl  of  leaves  (the  calyx  and  corolla).  This  subdivision  embraces 
most  garden  vegetables,  cultivated  flowers,  fruit-trees,  the  locust,  ash, 
elder,  etc. 

Animal  Kingdom, 

all  organisms  requiring  proteid  food. 

I.  Protozoa  {first  life).  The  simplest  animals;  mostly  micro- 
scopic; consisting  of  a  single  cell,  with  or  without  nucleus;  no 
stomach  or  organs,  as  the  amosba  (Fig.  123)  and  other  animalcules, 
the  radiolarians  and  foramenifers  found  in  the  oceanic  oozes,  infu- 
sorians,  etc. 


Fig.  130. 


322 


PHYSICAL    GEOGRAPHY. 


Fig.  131. 


2.  Porifera  {pore  bearers).  Animals  having  many  cells  but  no 
special  organs.  These  animals  are  traversed  by  many  pores,  or 
cavities,  which  serve  the  purpose  of  a  simple 
stomach.  Though  possessing  no  fixed  symmetry 
of  form,  most  of  these  animals  secrete  a  stony 
or  horny  substance  from  their  food,  which  serves 
the  purpose  of  an  irregular  frame-work  or  skel- 
eton.    Such  animals  are  the  sponges  (Fig.  131). 

3.  Ccelenterata  {hollow  stomached).  Animals 
possessing  a  single,  distinct  stomach-cavity,  with 
a  body-cavity  extending  off  from  it.  In  this  cav- 
ity or  elsewhere  several  distinct  organs  appear; 
a  more  or  less  distinct  symmetry  of  form,  similar 
parts  of  the  body  being  usually  arranged  around 
a  center,  like  the  spokes  of  a  wheel  around  the 
hub.  Such  animals  are  the  hydras ;  medusa,  or 
jelly-fishes  (Fig.  132);  and  the  corals  (Fig.  68). 

4.  Echinodermata  {spiny  or  rough  skinned),  having  true  stomach, 
separate  from  another  body -cavity,  containing 
organs  answering  to  a  heart  and  nervous  system; 
radial  symmetry  like  the  preceding,  but  each  ray 
usually  consists  of  two  similar  halves,  placed  side 
by  side  (bilateral  symmetry).  Such  are  the  crin- 
oids,  star-fishes ;  sea-urchins  (Fig.  133),  and  sea- 
cucumbers. 

5.  Vermes  {worms).     Lowest  animals  possess- 
ing clearly  bilateral  symmetry;   stomach  divided 
into  various  special  parts  ;   body  composed  of  a 
series  of  rings  or  segments ;  distinct  head  containing  nervous  cen- 
ters (ganglia),  such  as  the  common  angle-worm. 

6.  Mollusca  {soft).  Soft, 
unsegmented  bodies,  bilat- 
erally symmetrical,  e  n  - 
veloped  by  a  leathery 
mantle,  which  usually  de- 
velops a  hard  shell-cov- 
ering, or  external  skel- 
eton ;  a  symmetrical  nerv- 
ous system,  consisting  of 
Fig.  133.  several  connected  nerve 


Fig.  13a. 


THE    VARIOUS    FORMS    OF    LIFE. 


323 


Fig.  134. 


bunches,    or  ganglia.      Such   are   the  clams,   oysters,   muscles,  and 
snails  (Fig.  134),  conchs,  cuttle-fishes,  the  nautilus,  etc. 

7.  Arthropoda  {jointed  feet).  Bilaterally  symmetrical  bodies 
composed  of  a  series  of  rings  or  segments,  each  of  which  bears  a 
pair  of  jointed  appendages,  or  limbs ;  a  well 
developed  and  symmetrical  nervous  system 
of  many  connected  ganglia.  The  lower  ar- 
thropoda are  the  crustaceans — barnacles,  lob- 
sters, (Fig.  135),  crabs,  etc.  The  higher  are 
the  insecta,  or  insects,  as  spiders,  myriapods, 
grasshoppers,  beetles,  flies,  moths,  butterflies, 
bees,  wasps,  ants,  etc. 

(8)  Vertebrata  {flexible).  Animals  possess- 
ing a  flexible  backbone  and  an  internal  skel- 
eton (see  Fig.  146) ;  a  true  brain  in  the  head 
(see  Fig.  148) ;  a  spinal  nerve  cord ;  and  a 
more  or  less  highly  specialized  nervous  system. 

The  subdivisions  of  this,  the  highest 
of  the  primary  groups  of  animals, 
are  (a)  fishes,  (b)  amphibians,  (frogs, 
toads,  etc.),  (c)  reptiles,  (snakes, 
lizards,  etc.),  (d)  birds,  and  (e) 
mammals  (the  breast),  or  animals 
which  feed  their  young  from  the 
breast.  The  highest  of  these  groups, 
the  mammals,  is  divided  into  three 
subgroups,  the  monotremes  or  lowest,  including  but  a  few  rare 
mammals  found  about  Australia  (Fig.  140),  having  a  body  and  skel- 
eton much  like  a  mole  or  hedgehog;  a  flat  bill  and  web  feet  like 
a  duck  or  alligator ;  and  which  hatch  their  young  from  an  egg  like 
a  bird  or  reptile.  The  next  higher  subgroup,  the  marsupials  (Figs. 
140,  145),  includes  a  greater  number  of  mammals,  but,  with  the  ex- 
ception of  the  opossum  of  America,  they  are  all  found  in  Australia 
and  the  neighboring  islands.  These  animals  bring  forth  their  young 
alive— that  is,  after  the  egg-envelope  is  broken — but  the  young  are 
brought  forth  in  such  an  imperfect  condition  that  for  some  time  after 
birth  they  are  carried,  attached  to  the  breast  of  the  mother,  in  a 
pouch,  or  fold,  of  skin  with  which  she  is  provided.  The  highest 
of  the  three  subgroups  include  all  the  rest  of  the  mammals,  from  the 
armadillos,  ant-eaters,  and  sloths,  which  have  the  lowest  and  sim- 
plest, to  man.  who  has  the  highest  and  most  complex,  organization. 


Fig.  135. 


324  PHYSICAL    GEOGRAPHY. 

By  comparing  the  characteristics  of  these  great 
primary  groups,  one  is  immediately  struck  by  the  fact  that 
in  both  the  vegetable  and  animal  kingdoms  the  organisms 
show  a  progressive  but  gradual  complication  of  structure, 
and  a  corresponding  specialization  of  function  in  their 
several  parts  or  organs ;  from  low,  independent  organisms 
without  definite  structure,  consisting  simply  of  minute 
masses  of  protoplasmic  jelly,  all  parts  of  which  possess 
equal  ability  to  perform  all  the  duties  essential  to  con- 
tinued life,  they  increase  to  forms  of  highly  complex  struct- 
ure, possessing  many  distinct  organs,  each  adapted  to  a 
special  function. 


Amoeba  Sponge  Crab 

(Protozoa)  (Porz/era)      (Arthropoda) 

Fig.  136.     Mature  amceba.and  germ-cells  of  successively  higher  groups. 

A  progressive  complication  of  structure  similar  to 
that  exhibited  by  the  great  primary  groups,  occurs  also 
during  the  very  early,  or  embryonic,  development  of  every 
individual  in  the  higher  groups  of  organisms.  As  already 
stated,  every  organism,  even  the  highest  and  most  com- 
plicated, is  at  first  a  simple  germ-cell  which  often  can  not 
be  distinguished  from  the  mature  single-celled  organisms 
of  the  lowest  groups  (Fig.  136).  The  embryo,  in  acquir- 
ing the  complicated  structure  peculiar  to  its  own  group, 
passes  in  succession  through  stages  in  which  it  closely  re- 
sembles more  nearly  mature  organisms  of  lower  groups. 

In  Fig.  137  is  shown  a  section  of  an  embryo  animal  in  each  of 
the  primary  groups,  at  the  gastrula  stage.  This  stage  occurs  just 
before  maturity  in  the  sponge,  but  at  successively  younger  periods 
in  the  higher  groups.  Fig.  138  shows  successively  younger  em- 
bryos from  successively  higher  groups  of  the  vertebrata. 


THE   VARIOUS    FORMS    OF    LIFE. 


325 


Sponge      Coral        Star-fish        Worm  Snail 

(Porifera)     (Calenter.)   (Echinoderm.)       (Vermes)  (MoUusca) 

S.  indicates  stomach  cavity. 

Fig.  137. 


Barna»le     Lancelet 
(Arthropoda)     ( Vertebrata) 


Trout 

Frog 

Turtle 

Chicken 

Cow 

(Fish) 

(Amphibian) 

(Reptile) 
Fig.  138, 

(Bird) 

(Mammal) 

The  fossil  remains  of  ancient  forms  of  life  indicate 
that  in  general  the  more  complex  and  specialized  organ- 
isms appeared  on  the  earth  at  successively  later  dates  in 
its  history.  The  oldest  rocks  in  which  metamorphism  has 
not  so  nearly  obliterated  the  fossils  as  to  render  their 
original  form  quite  unrecognizable,  are  those  of  the  Cam- 
brian period.  In  these  rocks,  fossil  thallogens  (sea-weeds) 
are  found,  but  no  forms  of  any  higher  vegetable  groups. 
Fossil  representatives  of  most  of  the  lower  animals  are 
also  found,  but  none  of  any  higher  group  than  arthropoda. 
It  is  not  until  late  in  the  succeeding  Silurian  period  that 
fossils  of  a  higher  group  of  plants  and  animals  (acrogens 
and  vertebrates — fishes)  first  appear.  But  fishes  are  the  low- 
est group  of  the  vertebrata.  At  successively  later  periods 
fossils  of  higher  groups  are  found  in  the  order  of  their 
complexity  (gyninosperm  plants  and  amphibians  and  rep- 
tiles), and  at  still  later  periods  the  still  higher  groups  of 


326 


PHYSICAL    GEOGRAPHY. 


birds,  mammals,  and  angiosperm  plants,  while  the  earliest 

remains  of  the  highest  organism,  man,  do  not  occur  until 

a  very  much  later  period. 

In  the  interval  between  the  appearance  of  successive  groups, 

fossil   forms,    now   extinct,   are   occasionally   found,   that    show    a 

structure  intermediate  be- 
tween the  two  groups. 
Thus,  the  fishes  are  linked 
to  the  amphibians,  and  the 
reptiles  to  the  birds,  by  ex- 
tinct fossil  forms  curiously 
blending  the  structures  of 
both  groups ;  while  the 
lowest  Australian  mam- 
mal, with  its  peculiar  web- 
footed  structure  and  curi- 
ous functions  of  laying 
eggs  and  yet  suckling  its 
young  when  hatched,  is 
sometimes  called  a  "  liv- 
ing fossil,"  being  really  a 
link  between  the  reptiles 
and  the  mammals. 

The  Development 
Theory. — Three  great 
Fig.  139.  facts  are  thus  presented 

to  us:  (1)  living  organisms  increase  more  or  less  gradually 
in  complexity  of  structure,  from  the  lowest  to  the  highest; 
(2)  there  is  a  gradual  complication  of  structure  of  each  or- 
ganism during  its  development,  from  a  simple  germ-cell 
resembling  the  lowest  forms,  through  stages  resembling 
successively  higher  forms ;  and  (3)  fossils  of  successively 
more  complicated  forms  of  life  appear  at  successively 
later  periods  of  time.  A  consideration  of  these  facts 
has  led  to  the  belief  that  life  first  appeared  upon  the 
earth  at  an  inconceivably  remote  period  in  the  past,  as  a 
single    kind    or   a   very   few   kinds   of  organisms   of  the 


Relative  age  of  various  groups 

Heiative 

length 

of  various 

Geological 

Periods 

Plants 

Animals 

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Archwan 

THE   VARIOUS    FORMS   OF    LIFE.  $27 

simplest  structure ;  and  that  as  age  succeeded  age,  different 
descendants  of  these  organisms  encountered  different 
changes  in  their  environment,  and  thus  were  differently 
altered  in  structure  and  function,  each  to  conform  to  its 
peculiar  surroundings,  each  generation  undergoing,  through 
its  power  of  adaptation,  a  very  slight  change  of  form,  and 
transmitting  it  to  the  next  generation,  through  the  power 
of  heredity.  As  countless  generations  followed  one  another, 
the  imperceptible  changes  in  each  gradually  produced  or- 
ganisms differing  widely  in  both  structure  and  habit,  or 
function,  from  their  remote  ancestors ;  and  as  the  surround- 
ings of  different  organic  groups  differed  greatly,  so  did  the 
resulting  organisms  differ  from  one  another  as  widely  as 
from  their  common  ancestors.  Thus,  it  is  conceived,  has 
arisen  not  only  the  infinite  variety  in  the  organic  forms 
inhabiting  the  world,  but  the  remarkable  adaptation  of 
each  form  to  its  special  surroundings,  or  environment. 

Great,  though  exceedingly  gradual,  changes  of  environment 
would  naturally  ensue  (i)  from  the  gradual  transference  of  organ- 
isms into  new  localities,  by  winds,  currents,  other  organisms,  or  their 
own  powers  of  movement;  (2)  from  gradual  changes  of  climatic 
or  other  conditions  resulting  directly  or  indirectly  from  the  cooling 
of  the  planet,  the  slow  movements  of  the  earth's  crust,  or  the 
equally  slow  effects  of  erosion ;  and  (3)  from  the  sharp  competition 
for  food  and  other  necessities  (air,  moisture,  light,  etc.),  resulting  from 
the  continued  multiplication  of  organisms  in  the  same  locality.  It  is 
probable  that  such  competition  has  tended  more  than  any  other  one 
cause  to  produce  the  endless  variety  and  the  progressive  complexity 
of  organic  forms ;  for,  where  competition  is  sharpest,  only  those 
organisms  that  are  most  perfectly  fitted,  or  adapted,  to  their  sur- 
roundings get  a  sufficiency  of  the  requisites  of  life,  while  the  less 
perfectly  adapted  organisms  have  to  migrate  or  perish.  The  favor- 
able peculiarities  of  the  survivors  are  inherited  by  their  descend- 
ants, and  in  a  few  generations  become  common.  Then  individuals 
possessing  some  new  peculiarity  become  the  favored  and  successful 
form. 


CHAPTER  XXIII. 
DISTRIBUTION   OF   LIFE. 

/  will  plant  in  the  wilderness  the  cedar,  the  acacia  tree,  and  the  myrtle,  and 
the  oil  tree ;  I  will  set  in  the  desert  the  fir  tree,  the  pine,  and  the  box  tree  together : 
that  they  may  see,  and  know,  and  consider,  and  understand  together,  that  the  hand 
of  the  Lord  hath  done  this.— Isaiah  xli  :  19,  20. 

In  general,  the  number  of  all  forms  of  life  decreases 
with  the  temperature  and  moisture  of  the  climate.  Thus, 
in  the  equatorial  regions,  where  heat  and  moisture  are 
great  and  continuous  throughout  the  year,  the  luxuriance 
of  both  animal  and  vegetable  life  is  astonishing.  Dense, 
continuous,  and  evergreen  forests  are  the  striking  feature 
of  the  vegetation.  The  trees  not  only  stand  very  close 
together,  but  their  trunks  and  branches  are  entwined  with 
huge  climbing  vines,  which,  stretching  from  tree  to  tree, 
and  interlacing  with  tall,  tree-like  ferns,  grasses,  and  other 
plants,  present  an  almost  solid  mass  of  vegetation  often 
quite  impassable  by  man.  It  forms  a  congenial  home, 
however,  for  myriads  of  animal  forms — mammals  in  great 
variety,  birds  of  gorgeous  plumage,  reptiles  of  many 
kinds,  toads  and  frogs,  snails  and  other  land  shells,  to- 
gether with  hosts  of  butterflies,  moths,  beetles,  and  other 
insects. 

In  passing  from  such  regions  to  polar  latitudes,  one 
may  observe  that  the  forests  gradually  become  more  open, 
and  the  undergrowth  less  dense,  while  the  animal  forms 
become  less  numerous  and  less  varied.  As  latitudes  are 
reached  in  which  the  difference  between  winter  and  sum- 
(328) 


DISTRIBUTION    OF    LIFE.  329 

mer  temperatures  is  more  marked,  the  broad-leafed  plants 
that  were  evergreen  nearer  the  equator  become  deciduous, 
that  is,  they  lose  their  leaves  on  the  approach  of  winter; 
while  almost  the  only  remaining  evergreens  have  needle 
leaves,  like  the  pines,  or  plate -like  foliage,  like  the 
cedars.  This  change  in  vegetation  is  accompanied  by  a 
corresponding  change  in  the  form  and  habits  of  animals. 
With  the  seasonal  stoppage  of  plant  growth,  many  of  the 
animals  are  forced  to  migrate  into  warmer  latitudes  in 
search  of  food ;  others  are  adapted  to  pass  the  winter  in  a 
nearly  unconscious  state  in  some  secure  retreat,  under- 
ground, in  hollow  trees,  or  elsewhere;  while  many  take 
on  a  thicker  covering  of  hair,  fur,  or  feathers  at  the  ap- 
proach of  winter. 

In  still  higher  latitudes,  even  such  hardy  trees  as 
pines,  firs,  larches,  and  spruces  can  not  withstand  the  cold 
winters;  all  trees  and  even  tall  shrubs  disappear,  and  in 
the  polar  lands  only  such  low  plants  can  mature  their 
seeds  during  the  short  growing  season  as  can  be  safely 
covered  by  the  snow  during  the  long  winter,  such  as 
mosses,  lichens,  and  strangely  dwarfed  and  stunted  shrubs. 
The  animals  of  these  cold  latitudes  are  comparatively  few, 
and,  like  the  plants,  are  specially  adapted  to  withstand  the 
severe  climate. 

The  low  northern  margins  of  both  America  and  Euro-Asia  lie 
partly  in  this  cold  region,  and  the  ground  is  frozen  to  a  great  depth. 
Only  a  thin  layer  is  thawed  during  the  short  summer.  The  water 
from  the  melting  snow,  unable  to  penetrate  the  frozen  ground  be- 
neath, converts  the  flat  region  into  a  great  morass,  or  tundra,  in 
whose  shallow  soil  the  mosses,  lichens,  and  other  low  forms  of  vege- 
tation peculiar  to  the  region  find  sufficient  foothold. 

Variation  with  Altitude. — A  very  similar  series  of 
changes  in  the  organic  world  may  be  observed  in  ascend- 
ing a  very  lofty  mountain  range  in  the  torrid  zone.  As 
the  temperature  falls  with  the  ascent,  the  rank  vegetation 


330  PHYSICAL    GEOGRAPHY. 

and  exuberant  life  of  the  lowlands  are  replaced  by  organic 
forms,  which  remind  one  of  those  to  be  found  in  the  low- 
lands of  colder  latitudes.  Finally,  an  altitude  is  reached 
where  tall  tree-growth  ceases,  and  is  succeeded  by  a 
region  in  which  the  vegetation  consists  only  of  mosses, 
lichens,  and  stunted  shrubs.  As  the  vicinity  of  the  snow 
line  is  reached  even  these  fail,  and  all  forms  of  life  are  left 
below. 

Effects  of  Moisture  —  Forests. — But  the  organic 
forms  do  not  depend  upon  temperature  alone.  Forests, 
with  their  peculiar  forms  of  animals,  occur  only  where  the 
rain-fall  is  abundant  throughout  the  growing  season.  In 
regions  where  the  rain-fall  is  but  moderate,  or  very  unequal 
at  different  seasons,  the  forests  are  replaced  by  the  low  veg- 
etation peculiar  to  open  meadows,  or  pastures. 

Open  Meadows. — Plains  having  this  form  of  vegeta- 
tion are  called  prairies,  steppes,  llanos,  pampas,  and  cam- 
pos  in  different  regions.  With  the  change  of  vegetable 
forms  from  forests  to  meadows,  a  corresponding  change 
takes  place  in  the  animal  forms.  Those  common  in  the 
forest  are  rarely  seen,  but  are  replaced  by  other  forms 
better  suited  to  the  open  country  and  the  altered  condi- 
tions of  climate. 

The  meadow  vegetation  may  be  evergreen  in  low  latitudes,  where 
the  annual  temperature  is  uniform,  if  the  rain-fall  is  equally  distrib- 
uted through  the  year  ;  but  where  precipitation  is  very  intermittent, 
the  vegetation  is  burned  crisp  and  brown,  and  killed  to  the  roots  by 
the  great  heat  in  the  dry  season.  Thus,  in  the  llanos  of  Venezuela, 
in  the  warm  valleys  of  California,  in  parts  of  India,  and  in  many 
other  regions,  the  surface  is  covered  with  verdure  during  the  wet 
season,  but  assumes  the  parched  aspect  of  a  desert  during  the  dry 
season. 

Deserts. — In  still  other  regions,  where  the  rain-fall  is 
very  slight,  vegetation  of  any  kind  is  scanty  or  entirely 
wanting,   and  a  true   desert  is  the   result.      The  animal 


DISTRIBUTION   OF    LIFE.  33 1 

forms  of  deserts  must  manifestly  be  very  different  from 
those  of  forests  or  meadow  lands,  not  only  because 
of  the  scarcity  of  water  to  drink,  but  because  of  the 
dearth  of  vegetable  matter  which,  directly  or  indirectly, 
affords  food  to  all  animals.  Hence,  desert  animals  must 
be  adapted  not  only  to  traverse  great  distances  in  order  to 
collect  enough  food  from  the  scanty  vegetation,  but  also 
adapted  to  escape,  by  flight,  or  strength,  or  concealment, 
from  other  animals  which  subsist  on  animal  food. 

The  close  relation  between  the  rain-fall  and  the  character  of  veg- 
etation, may  be  appreciated  by  comparing  the  chart  of  Vegetation 
Regions  (page  332),  with  that  of  the  Rain-fall  (page  76).  It  is  seen 
that  all  deserts  correspond  to  regions  of  very  light  rain-fall,  and 
that  the  regions  of  heaviest  forests  are  in  regions  of  heaviest 
rain-fall.  But  it  is  also  seen  that  the  northern  regions  of  open  for- 
ests in  both  America  and  Euro-Asia  lie  mostly  in  regions  of  mod- 
erate rain-fall;  while  in  both  South  America  and  Africa,  certain 
regions  of  heavy  rain-fall  have  only  meadow-land  vegetation. 
These  apparent  anomalies  are  the  result  of  inequalities  in  the 
annual  distribution  of  rain-fall.  In  low  latitudes,  the  high,  uniform 
temperature  permits  plant  growth  during  the  entire  year;  but  where 
wet  and  dry '  seasons  alternate,  the  plants  are  killed  to  the  roots 
during  the  dry  season,  and  only  the  low  herb  or  shrub  vegetation  of 
meadows  can  mature  during  the  wet  season.  In  high  latitudes, 
however,  the  great  annual  variation  of  temperature  adapts  some 
plants  to  pass  a  great  part  of  the  year  (winter)  in  a  dormant  condi- 
tion, and  to  begin  growth  in  the  spring  where  they  left  off  growing 
in  the  autumn.  In  such  latitudes,  if  a  small  annual  rain -fall 
occurs  mostly  during  the  short  growing  season,  it  produces  the 
same  effect  upon  vegetation  as  a  larger  annual  rain -fall  in 
latitudes  where  the  growing  season  is  longer.  To  a  certain  alti- 
tude, mountain  ranges  generally  receive  a  heavier  rain-fall  than  the 
adjacent  lowlands,  and  for  this  reason  are  usually  forest-clad,  though 
they  rise  from  the  midst  of  extensive  treeless  plains,  or  even  from 
deserts  (page  311). 

Distribution  of  Different  Kinds  of  Life. — While  the 
general  form  and  habit  of  plants  and  animals  are  thus 
largely  determined  by  the  climate,   there   are  many  pe- 


DISTRIBUTION    OF    LIFE.  333 

culiarities  which  climate  alone  will  not  account  for,  when 
the  kinds  of  plants  or  of  animals  in  one  region  are  com- 
pared with  those  in  another.  If  climate  were  the  sole 
cause  of  the  present  distribution  of  plants  and  animals, 
we  should  expect  to  find  the  organisms  of  different  regions 
having  very  similar  climates  more  closely  related  to  each 
other  than  those  living  in  regions  having  very  dissimilar 
climates.      But  very  frequently  this  is  not  the  case. 

Many  of  the  organisms  of  semi-tropical  Florida  and  Georgia 
have  nearer  relatives  in  the  Arctic  regions  of  both  America  and 
Euro-Asia,  than  in  the  closely  neighboring  Bahama  Islands,  Cuba, 
or  Yucatan,  where  the  climate  is  so  much  more  nearly  the  same. 
Parts  of  equatorial  South  America  and  west  Africa  are  almost 
identical  in  climate,  and  yet  differ  widely  in  their  assemblages  of 
plants  (flora),  and  of  animals  (fauna).  The  flora  and  fauna  of 
Euro-Asia  north  of  the  Himalaya  Mountain  system  differs  less  from 
that  of  North  America  than  it  does  from  either  that  of  Africa  south 
of  the  Sahara,  or  of  Asia  south  of  the  Himalaya  r3 nge.  These 
latter  regions,  though  often  similar  in  climate,  differ  greatly  from 
each  other  in  flora  and  fauna,  while  many  of  the  plants  and  most  of 
the  animals  of  Australia,  which  closely  resembles  parts  of  south 
Africa  in  climate,  have  no  near  relatives  in  that  continent. 

In  general,  every  region  of  the  land  that  is  broadly 
separated  from  surrounding  regions  by  great  differences  of 
climate,  or  strongly  marked  physical  barriers  to  the  pas- 
sage of  plants  and  animals,  such  as  the  sea,  lofty  mountain 
chains,  or  wide  areas  of  desert,  differs  to  a  greater  or  less 
extent  from  the  surrounding  regions  in  flora  and  fauna. 
The  amount  of  this  difference  is  roughly  proportional  (i) 
to  the  completeness  of  the  separation,  or  isolation,  of  the 
regions,  and  (2)  to  the  length  of  time  they  have  been 
separated ; — the  difference  being  greatest  where  the  isolation 
is  greatest,  and  has  endured  for  the  longest  time. 

The  development  theory  explains  why  this  should  be  the  case. 
The  environments  of  organisms  in  two  regions  are  never  exactly 
the  same,  and  each  environment  is  constantly,  though  perhaps  very 


(334) 


DISTRIBUTION    OF    LIFE.  335 

slowly,  changing,  while  the  organisms  and  their  descendants  in  ei 
region  are  constantly  and  gradually  changing  alsof-each  organism 
adapting  itself  to  its  own  special  environment.  Hence,  if  very 
similar  and  closely  related  organisms  were  placed  in  two  regions 
separated  from  each  other  by  impassable  barriers,  their  remote  de- 
scendants would  inevitably  be  very  different,  not  only  from  their 
similar  ancestors,  but  from  each  other ;  whereas,  if  no  barriers 
isted,  the  descendants  in  either  region  would  constantly  mingle  with 
those  in  the  other  region,  and  the  related  organisms  would  thus  tend 
to  continue  similar  in  the  two  regions,  though  in  both  they  might 
eventually  develop  features  which  their  ancestors  did  not  possess. 

Primary  Biological  Regions. — A  comparison  of  the 
floras  and  the  faunas  of  different  lands  indicates  that  the 
continental  plateau  may  be  divided  into  six  great  biological 
or  life  regions,  each  characterized  by  the  abundance  of 
certain  kinds  of  plants  and  animals  belonging  to  great  or- 
ganic groups  that  are  not  represented  at  all,  or  not 
nearly  so  abundantly,  in  any  other  region.  Each  geo- 
graphical grand  division,  with  one  exception,  corresponds 
roughly  with  a  single  primary  biological  region.  Thus, 
we  have  (i)  the  South  American  region;  (2)  the  North 
American  region;  (3)  the  Euro- Asian  region;  (4)  the 
African  region;  (5)  the  Australian  region;  and  (6)  the 
Oriental  region,  which  comprises  the  greater  part  of  the 
Malay  Archipelago  and  the  portion  of  the  main-land  of 
Asia  south  and  east  of  the  Himalaya  Mountains. 

Where  the  primary  regions  are  not  separated  by  the  great 
oceanic  depressions,  their  boundaries  generally  overlap  each  other, 
producing  transitional  regions,  in  which  the  characteristic  plants 
and  animals  of  both  the  adjacent  primary  regions  are  found  to  a 
greater  or  less  extent.  There  are  at  least  four  transitional  regions : 
(1)  the  Mexican  region,  embracing  Mexico  and  the  hot  desert 
region  of  the  south-west  United  States ;  (2)  the  Mediterranean  re- 
gion, embracing  both  coasts  of  that  sea  and  the  continuous  desert 
territory  on  the  south  and  east,  from  the  Atlantic  to  the  valley  of 
the  Indus  and  the  Hindoo  Koosh  Mountains  ;  (3)  the  Chinese  region 
from  the  Nan  Ling  (mountains)  on  the  south,  to  the  Khin  Gan  system 
P.  G.-X9. 


336  PHYSICAL    GEOGRAPHY. 

an  the  north ;  and  (4)  the  Papuan  region,  embracing  Celebes,  Papua, 
or  New  Guinea,  and  the  smaller  neighboring  islands  of  the  Malay 
Archipelago.  On  the  chart  (page  334)  the  approximate  boundaries 
and  the  relative  positions  of  the  primary  and  transitional  regions  of 
the  continental  plateau  are  indicated.  It  will  be  observed  that  these 
transitional  regions  occupy  territory  in  which  there  is  a  marked  tran- 
sition in  climate,  or  well  marked  peculiarities  of  surface,  such  as 
high  mountain  ranges,  broad  deserts,  or  wide  areas  of  the  sea,  ) 
which  constitute  a  succession  of  barriers  to  the  free  passage  of  or- 
ganic forms  between  the  adjacent  primary  regions. 

A  seventh  biological  region  may  be  said  to  embrace 
all  the  oceanic  island  groups,  for  although  the  various 
groups  differ  from  one  another  in  flora  and  fauna,  still  they 
all  possess  in  common  certain  striking  peculiarities  when 
compared  with  the  continental  regions,  and  these  peculi- 
arities throw  some  light  on  the  general  distribution  of 
life. 

Characteristics  of  the  Island  Region, — Of  all  regions, 
that  of  the  oceanic  islands  is  most  completely  isolated,  both 
as  to  space  and  time.  Each  island  group  is  not  only  sep- 
arated from  the  continents  by  a  great  width  of  ocean,  but 
all  evidence  indicates  that  it  has  always  been  so  separated. 
Hence,  we  should  expect  that  each  island  group  would  be 
peopled  with  species  of  plants  and  animals  found  nowhere 
else  in  the  world,  and  this,  in  general,  is  the  case.  • 
Though  peculiar  in  species,  the  island  organisms  almost 
always  show  a  greater  or  less  resemblance  in  internal 
structure  to,  and  are  hence  the  near  or  remote  kindred  of, 
organisms  inhabiting  the  nearest  continent — though  this 
may  be  hundreds  of  miles  distant —  and  this  resemblance 
is  greatest  in  islands  where  strong  prevailing  winds  and 
oceanic  currents  move  directly  from  the  continents  toward 
the  islands.  The  bearing  of  these  facts  becomes  apparent, 
when,  upon  closer  examination,  it  is  found  that  all  the 
native  plants  and  animals  of  oceanic  islands  are  of  kinds 


DISTRIBUTION    OF    LIFE.  337 

which,  at  some  stage  of  life,  are  specially  adapted  for  a 
wide  dispersal,  either  through  the  agency  of  winds  or  of 
ocean  currents.  These  facts,  in  connection  with  the  ab- 
sence of  fossil  remains  of  very  ancient  forms  of  life,  render 
it  nearly  certain  that  the  oceanic  islands  are  peopled  by 
such  stray  forms  of  continental  plants  and  animals  as  find 
a  congenial  environment  after  being  cast  upon  their  shores 
by  the  direct  or  indirect  agency  of  the  winds  and  oceanic 
currents ;  and  that  the  amount  of  peculiarity  in  any  special 
form  of  island  life  is  proportional  to  the  length  of  time 
since  the  arrival  of  its  first  island  ancestors  or  any  similar 
forms. 

Among  the  most  common  plant  groups  of  oceanic  islands  are  the 
ferns,  grasses,  and  sedges ;  some  of  the  palms ;  the  group  including 
the  thistle  and  dandelion ;  and  that  to  which  beans,  peas,  and  the 
locust  belong.  The  seeds  of  all  these  plants  are  lighter  than  water ; 
they  retain  vitality  long  after  detachment  from  the  parent  stem  ;  and 
either  (i)  are  very  minute  and  furnished  with  a  wing-like  "down," 
which  adapts  them  for  transportation  by  the  winds ;  or  (2)  are  envel- 
oped in  a  water-proof  shell,  or  pod,  which  enables  them  to  float  safely ; 
or  (3)  in  a  bur,  which  will  adhere  to  the  plumage  of  birds;  or  (4)  are 
so  protected  in  a  small  fruit  or  berry,  which  birds  swallow  whole,  that 
the  living  seed-germ  may  be  transported  in  the  bird's  stomach  over 
great  distances.  There  are  other  groups  of  plants  represented  on 
oceanic  islands,  but  seldom  or  never  any  having  heavy  or  perisha- 
ble seeds.  The  native  animal  groups  seem  to  be  exclusively  con- 
fined to  birds,  insects,  bats,  certain  land  mollusks,  and  certain 
reptiles.  The  first  three  can  fly.  Mollusks  are  very  tenacious  of 
life ;  some  can  seal  their  shells  up  water-tight,  and  thus  float  safely 
for  many  weeks ;  others  reach  islands  attached  to  floating  drift- 
wood ;  while  the  living  eggs  of  still  others  have  been  found  in  the 
mud  attached  to  the  feet  of  birds.  The  eggs  of  island  reptiles,  gen- 
erally lizards,  are  doubtless  thus  transported,  attached  to  the  feet  of 
birds  or  bats.  The  absence  from  oceanic  islands  of  all  native  four- 
footed  mammals,  all  amphibians  (toads,  frogs,  etc.),  and  all  fresh- 
water fishes,  renders  the  fauna  strikingly  peculiar.  The  influence 
of  the  direction  of  winds  and  currents  upon  the  amount  of  pecu- 
liarity of  island  life  is  well  shown  by  comparing  the  life-forms  of  the 


338  PHYSICAL  GEOGRAPHY. 

Bermudas,  Azores,  and  Galapagos  islands  respectively,  with  those  of 
the  nearest  continental  region,  from  which  they  evidently  were  orig- 
inally peopled.  The  constant  Gulf  Stream  and  regular  winter  west 
winds  so  frequently  carry  American  forms  of  life  to  the  Bermudas, 
that  the  island  breeds  are  kept  similar  to  the  continental  breeds, 
and  few  distinct  species  are  found.  The  regular  winds  and  currents 
move  from  the  Azores  toward  Europe,  and  stray  life-forms  from 
Europe  are  brought  to  the  islands  only  occasionally  by  the  intermit- 
tent cyclonic  or  storm  winds,  and  hence  time  is  allowed  for  many 
species  to  become  peculiar.  The  Galapagos  Islands  are  nearer  to 
a  continent  than  either  of  the  other  groups,  but  lie  in  the  calm, 
stormless,  equatorial  region  of  the  Pacific,  and  immigrants  arrive  so 
seldom  that  all  the  organisms  belong  to  distinct  local  species,  ex- 
cepting the  extensive  migrator,  the  rice-bird,  or  bobolink. 

No  one  of  the  continental  regions  is  so  completely 
isolated  from  all  others  as  the  island  region,  and  between 
none  of  them  are  the  present  barriers  nearly  as  permanent 
as  the  great  sea-barrier  which  isolates  the  oceanic  islands; 
for,  as  a  result  of  the  constant  but  gradual  upheaval  or  de- 
pression of  portions  of  the  continental  plateau,  the  conti- 
nents and  grand  divisions  have  been  both  more  closely 
united,  and  more  effectually  separated,  than  they  are  at 
present.  Climatic  barriers  have  also  changed  materially 
on  the  continental  plateau,  for  such  alterations  of  level, 
through  their  effect  upon  the  direction  of  oceanic  and 
atmospheric  currents,  have  been  largely  instrumental  in 
bringing  about  such  vast  changes  of  climate  in  the  past  as 
are  evidenced  by  the  occurrence  in  arctic  lands  of  the 
fossils  of  tropical  plants  and  animals ;  and  by  the  traces  of 
continental  glaciers,  which  indicate  the  existence  of  an 
arctic  climate  as  far  south  in  the  United  States  as  the 
Ohio  valley.  It  has  thus  been  possible  in  the  past  for 
representatives  of  all  the  great  groups  of  plants  and  ani- 
mals to  migrate  to  and  fro  between  continental  regions 
that  are  now  separated  by  impassable  climatic  or 
physical   barriers.     Hence,   these  regions     differ    among 


DISTRIBUTION    OF    LIFE. 


339 


themselves  in  organic  forms  much  less  than  they  collec- 
tively do  from  the  island  region,  and  the  difficulty  of  ac- 
counting for  the  existing  differences  and  resemblances  of 
flora  and  fauna  is  vastly  increased,  and  frequently  rendered 
quite  impossible.  ' 

The  Australian  region  is  by  far  the  most  peculiar  of 
the  continental  regions,  since,  in  addition  to  a  great  num- 
ber of  peculiar  kinds  of  plants,  its  mammals,  with  scarcely 
an  exception,  belong  to  two  small  and  very  peculiar  sub- 
groups (Fig.  140),— the  monotremes,  or  egg-layers,  which 
are   found   in   no   other   region,    and   the   marsupials,    or 


Duck-Mole.  Kangaroo.  Brush-turkey.     Lmu. 

Fig.  140.— Characteristic  Animals  of  Australia. 

pouched  animals.  These,  though  represented  by  the 
kangaroo  and  many  variously  adapted  forms  in  Australia, 
are  not  represented  by  living  varieties  in  any  other  region 
excepting  the  two  most  distant  from  Australia: — a  few 
kinds  of  this  subgroup  (about  twenty  varieties  of  opossum) 
being  found  in  South  America,  and  two  varieties  occurring 
in  the  United  States. 

Among  the  more  characteristic  and  peculiar  plants  of  Australia 
are  the  leafless  beef-wood  trees,  the  very  numerous  and  generally 
leafless  varieties  of  the  acacia;  the  great  eucalyptus  trees,  whose 
leaves  grow  with  their  edges  to  the  sky,  so  that  they  cast  but  little 
shade ;  the  heather-like  epacris ;  and  (especially  in  New  Zealand) 
many  filmy-  and  tree-ferns.  The  characteristic  plants  are  most  nu- 
merous in  southern  Australia,  while  in  the  north  they  are  mixed 


34Q 


PHYSICAL    GEOGRAPHY. 


with  palms  and  other  tropical  plants,  identical  or  nearly  so  with  the 
plants  of  the  Malay  Archipelago  and  south-eastern  Asia,  from  which 
region  they  have  doubtless  reached  Australia,  in  comparatively 
recent  times.  The  peculiar  birds  of  Australia  include  piping  crows, 
honey-suckers,  lyre-birds,  cockatoos,  gayly  colored  pigeons,  brush- 
turkeys  or  mound-builders,  and  the  almost  wingless  emus  and  cas- 
sowaries, whose  nearest  relatives  are  the  ostriches  of  Africa  and  the 
rheas  of  South  America. 


Sloth. 

Rhea. 

Prehensile-tailed  Monkey, 


Fig.  141.— Characteristic  Animals  of  South  America. 


The  South  American  region,  though  its  flora  and 
fauna  are  among  the  richest  and  most  varied  in  the  world, 
probably  ranks  after  the  Australian  as  the  most  peculiar 
region.  The  striking  characteristic  of  this  region  is  the 
preponderance  in  its  fauna  of  lowly  organized  types  (Fig. 


DISTRIBUTION    OF    LIFE.  34 1 

141).  The  marsupials  (opossums),  edentates  (sloths,  ar- 
madillos, and  ant-eaters),  and  such  rodents  as  the  cavys, 
guinea-pigs,  and  agoutis,  form  the  majority  of  the  mam- 
mals, while  the  more  highly  organized  carnivora  and 
hoofed-animals  are  not  only  exceedingly  deficient,  but  are 
smaller  than  their  kindred  in  the  old  world ;  the  tiger, 
lion,  rhinoceros,  camel,  and  hog  of  the  old  world,  being 
represented  by  the  respectively  smaller  jaguar,  puma, 
tapir,  llama,  and  peccary  in  South  America.  The  same 
relatively  low  type  of  organization  characterizes  the  South 
American  monkeys  and  birds.  The  nostrils  of  the  former 
face  outward  instead  of  downward,  and  most  of  them 
are  prehensile-tailed ;  and  an  unusually  large  proportion  of 
the  birds  are  songless;  while  the  tinamous  and  rhea,  and 
the  curassow,  are  allied  respectively  to  the  lowly  organized 
ostriches,  and  to  the  brush-turkeys  of  Australia. 

Among  the  hundreds  of  peculiar  plants  of  the  South  American 
region  may  be  mentioned  the  mahogany,  rose-wood,  and  logwood, 
the  cinchona  or  Peruvian  bark  tree,  and  plants  yielding  India  rubber, 
many  spices,  balsams,  and  varnishes,  a  great  variety  of  laurels, 
bean-trees,  and  palms,  many  cacti  and  orchids  or  air-plants  (one  of 
the  latter  yielding  the  vanilla  bean),  peculiar  varieties  of  bananas, 
tree-grasses  (bamboo),  and  tree-ferns,  and  the  peculiar  varieties  of 
the  nightshade  family,  such  as  Cayenne  pepper,  the  potato,  and  to- 
bacco, while  Indian  corn  and  the  tomato  are  probably  descendants 
of  plants  native  in  this  region. 

The  African  and  Oriental  regions  possess  marked 
peculiarities  by  which  each  may  be  distinguished  from  all 
the  other  regions;  but  the  flora  and  fauna  are  more  com- 
plicated, highly  organized  forms  of  life  are  more  numer- 
ous, and  the  difficulty  of  accounting  for  the  present 
distribution  is  vastly  greater  than  in  the  two  preceding 
regions.  The  African  region  is  the  more  peculiar.  It 
differs  markedly  from  South  America  and  Australia  in 
the  great  development  of  the  highly  organized  carnivora 


342 


PHYSICAL    GEOGRAPHY. 


'     /  "^jl^Jfa  "A"*     """N 


lard-vark. 

Pangolin. 

Ostrich. 

Giraffe. 

Gnu. 

Zebra. 

Lion. 

Hippopotamus. 

Gorilla. 

Fig.  142.— Characteristic  Animals  of  Africa. 

and  hoofed-animals,  especially  antelopes,  of  which  more 
kinds  (80  or  90)  are  found  than  in  any  other  region.  Be- 
sides these,  the  hippopotamus,  giraffe,  zebra,  quagga,  and 
wild  ass  (the  ancestor  of  the  domestic  animal)  are  found 
nowhere  else.  Such  widely  spread  animals,  however,  as 
bears,  moles,  deer,  sheep,  and  goats  are  completely  absent 
from  the  African  region.  With  these  marked  differences, 
this  region  more  than  any  other  resembles  South  America 
in  possessing  a  moderate  number  of  the  lowly  organized 
edentate  animals  (the  ant-eating  aard-varks  and  pango- 
lins). The  carnivorous  animals  include  the  lion,  leopard, 
panther,  several  kinds  of  hyenas,  the  jackal,  aard  wolf,  and 


DISTRIBUTION    OF    LIFE. 


343 


/^>M%'^ 


Jungle-Bear. 

Gayal. 
Orang-outang. 


Tiger. 

Muntjac. 

Silver  Pheasant. 


Tapir. 

Chevrotain. 

Peacock  Pheasant. 


Fig.  143.— Characteristic  Animals  of  the  Oriental  Region. 

a  great  variety  of  civet  cats.  Most  of  these,  as  well  as  near 
relatives  of  the  African  elephant  and  rhinoceros,  are  also 
found  in  south-eastern  Asia,  and  are  characteristic  of  the  Ori- 
ental, rather  than  of  the  African,  region.  The  monkey  tribe 
of  the  two  regions  embraces  the  man-like  apes — the  gorilla 
and  chimpanzee  of  Africa,  and  the  orang-outang  of  Farther 
India, — besides  numerous  baboons,  true  monkeys,  and  the 
peculiar  half  monkeys,  or  lemurs.  Among  the  peculiar 
African  birds  may  be  mentioned  the  true  ostrich,  the 
serpent-eating  secretary-bird,  many  guinea-fowls,  vulture- 
crows,   plaintain-eaters,   crested  tourocos,  and  colies. 

The    Oriental    region    is   characterized   by  its   orang- 
outangs, and   by   a   greater   development   of    carnivorous 


344  PHYSICAL    GEOGRAPHY. 

animals  than  Africa;  for,  in  addition  to  the  lion,  leopard, 
hyena,  etc.,  it  possesses  the  tiger  and  ounce.  It  differs 
from  Africa,  too,  in  having  bears,  several  kinds  of  deer 
(muntjac,  chevrotain,  etc.),  wild  cattle  (the  gayal  and 
others),  wild  hogs,  and  a  tapir  closely  related  to  the 
South  American  animal.  This  region  is  the  head-quarters 
of  true  mice  and  squirrels,  and  contains  some  very  peculiar 
flying  lemurs.  Among  its  characteristic  birds  are  babbling- 
thrushes,  hill-tits,  green  bulbuls,  numerous  crows  and  horn- 
bills,  and  a  great  variety  of  magnificent  pheasants,  including 
the  peacock  and  jungle-fowl,  from  which  the  domestic 
chickens  are  descended. 

The  African  flora  embraces  the  oil-palm,  the  great  baobab,  eu- 
phorbias, bignonias,  and  tamarinds,  together  with  many  varieties  of 
laurel,  fig,  myrtle,  acacia,  and  mimosa  in  the  dense  forest  region  of 
the  west.  On  the  more  open  eastern  tablelands,  tall  grasses,  sedges, 
and  the  coffee  tree  are  characteristic  plants ;  while  the  remarkably 
rich  flora  of  the  south  includes  a  great  variety  of  heather,  fig- 
marigolds,  and  aloes,  together  with  some  close  allies  of  characteristic 
Australian  and  South  American  plants.  -  The  flora  of  the  Oriental 
region  as  a  whole  has  fewer  distinctive  features  than  that  of  Africa 
or  South  America.  The  region  is  relatively  small,  and  has  a  great 
climatic  range,  from  the  perpetual  snows  of  the  Himalayas  to  the 
equatorial  lowlands  of  Java,  and  hence  many  plants  from  adjacent  re- 
gions can  reach  congenial  environments  within  its  boundaries.  Along 
the  Himalayas  many  pines,  junipers,  yews,  cedars,  and  some  oaks 
occur.  In  the  dry  districts  of  north-west  India  many  acacias  and 
the  tamarisk  are  found.  In  the  moist  forest  regions  of  the  south 
and  south-east,  pitcher-plants,  wood-oil  trees,  custard-apples,  man- 
goes, numerous  palms,  cycads,  and  many  spice-yielding  plants 
abound,  while  teak,  toon,  sal,  ebony,  satin-wood,  sandal-wood,  and 
iron-wood  are  characteristic  timber  trees. 

The  Euro-Asian  and  North  American  regions, 
though  more  extensive,  differ  from  each  other  less  in  flora 
and  fauna  than  any  other  two  regions.  By  some  they  are 
classed  as  a  single  region,  but  minor  differences  seem  to 
warrant  their  division.     The  more  common  and  conspicu- 


DISTRIBUTION    OF    LIFE. 


345 


ous  animals,  such  as  the  various  wild  cats,  lynxes,  wolves, 
foxes,  weasels,  bears,  elk,  deer,  voles,  beavers,  squirrels, 
marmots,  and  hares,  are  identical,  or  very  similar,  in  the  two 
regions.  Even  the  grizzly  bear  of  the  Rocky  Mountains 
and  the  buffalo  (bison)  of  the  Great  Plains  are  thought  to 
be  but  slight  variations  of  the  European  brown  bear  and 
the  nearly  extinct  aurochs  of  west  Russia.  But  with 
these  there  are  in  each  region  numerous  less  conspicuous 
animals  not  found  in  the  other,  and  some  found  no- 
where else;    thus,  the  star-nosed  mole,   skunk,  raccoon, 


Chamois.  Ibex.  Wild  Boar. 

Hedgehog.  Camel.  Desman. 

Fig.  144.— Characteristic  Animals  of  Euro-Asia. 

puma  or  panther,  prong-horned  antelope,  Rocky  Mountain 
goat,  big  horn  sheep,  musk-ox,  musk-rat,  jumping-mouse, 
prairie-dog,  gopher,  tree-porcupine,  sewellel,  otter,  and 
opossum  occur  in  North  America  but  not  in  Euro-Asia, 
while  hedgehogs,  wild  boars,  dormice,  badgers,  camels, 
yaks,  saiga  antelopes,  and  nineteen  kinds  of  wild  sheep 
and  goats  occur  in  Euro-Asia  but  not  in  America. 

The  same  general  resemblance  with  numerous  special  differences 
characterizes  the  birds  and  plants  of  the  two  regions.     Eagles,  owls, 


346 


PHYSICAL    GEOGRAPHY. 


Big  Horn. 
Prairie-dog. 


Prong-horned  Antelope. 
Opossum. 


Fig.  145.— Characteristic  Animals  of  North  America. 

hawks,  crows,  thrushes,  wrens,  tits,  and  finches  occur  in  both,  but 
America  alone  possesses  humming-birds,  wild  turkeys,  turkey-buz- 
zards, blue  jays,  tanagers,  hang-nests,  and  mocking-birds ;  while 
Euro-Asia  is  peculiar  in  its  starlings,  magpies,  nightingales,  true  fly- 
catchers, partridges,  pheasants,  vultures,  etc.  Among  plants,  both 
regions  exceed  all  others  in  the  development  of  the  pine  family,  includ- 
ing the  pines,  larches,  spruces,  firs,  hemlocks,  cedars,  etc.;  and  of  the 
oak  family,  including  oaks,  chestnuts,  beeches,  hornbeams,  etc. 
The  ash,  elm,  sycamore,  walnut,  and  maple  are  also  charac- 
teristic plants,  as  are  also  numerous  gentians,  rushes,  prmroses, 
birches,  willows,  and  saxifrages.  The  American  region  is  peculiar 
in  its  asters,  golden-rod,  sequoias,  and  bald  cypresses,  while  Euro- 
Asia  shows  the  greater  development  of  heather,  roses,  olives, 
almonds,  etc. 

Taken  as  a  whole,  the  more  highly  organized  forms 
of  life  preponderate  on  the  land  masses  of  the  northern 
hemisphere — North  America  and  Euro- Asia, — the  great 
central  region  of  the  continental  plateau  (see  chart,  pages 
152,  153);  while  forms  of  land  life  of  lower  organization 
are  characteristic  of  the  extremities   of   the    continental 


DISTRIBUTION    OF    LIFE.  347 

plateau  which  project  into  the  southern  hemisphere — 
South  America,  south  Africa,  and  Australia.  Now,  when 
all  the  fossils  of  ancient  organisms  from  different  parts  of 
the  world  are  compared,  it  is  found  that  the  collection 
from  the  three  northern  regions  includes  not  only  the  evi- 
dent ancestors  of  the  forms  now  occupying  those  regions, 
and  of  the  more  highly  organized  forms  now  confined  to  the 
southern  hemisphere,  but  from  the  older  rocks  come  the 
fossil  ancestors  of  the  lowly  organized  forms  now  charac- 
teristic of  the  southern  land  masses. 

Not  only  do  the  fossils  prove  that  the  elephant,  rhinoceros,  and 
hippopotamus  were  once  far  more  abundant  in  Europe  than  they 
are  now  in  the  tropics,  but  they  also  prove  that  the  man-like  apes 
of  west  Africa  and  Malaya,  the  lemurs  of  Madagascar,  the  edentata 
of  South  America  and  Africa,  and  the  marsupials  of  Australia  and 
South  America  were  all  inhabitants  of  Euro-Asia  and  North  America 
at  the  beginning  of  the  tertiary  era  of  geological  time.  Though 
animal  forms  are  preserved  as  fossils  much  more  frequently  than 
vegetable,  still  the  indications  are  that  the  same  facts  are  as  true  of 
plants  as  of  animals. 

These  facts  indicate  that  at  least  during  the  vast 
length  of  time  that  has  elapsed  since  the  beginning  of  the 
tertiary  era,  and  probably  for  much  longer,  the  great  land 
masses  of  the  northern  hemisphere  have  occupied  substan- 
tially their  present  sites,  and  that  in  this  great  compact 
central  region  of  the  continental  plateau,  as  it  underwent 
the  long  series  of  changes  which  resulted  in  its  present 
physical  conditions,  all  the  successive  types  of  land  organ- 
isms gradually  developed,  from  ths  lowest  to  the  highest. 

In  the  southern  hemisphere,  thefe  appear  to  have 
been  three  smaller  but  equally  ancient  land  masses,  vary- 
ing gradually  in  extent,  but  always  keeping  distinct  from 
each  other,  and  occupying  roughly  the  sites  of  Australia, 
South  America,  and  south  Africa,  respectively.  From  time 
to  time  gradual  movements  of  the  earth's   crust   tempo- 


348  PHYSICAL    GEOGRAPHY. 

rarily  united  these  isolated  extremities  of  the  continental 
plateau  with  the  great  central  land  mass,  and  during  each 
connection,  which  may  have  lasted  thousands  of  years, 
the  various  forms  of  life  then  prevalent  in  the  central  con- 
tinents gradually  found  their  way  southward.  After  a 
longer  or  shorter  union,  the  gradual  subsidence  and  sub- 
mergence of  the  connecting  land  stopped  this  migration, 
and  a  period  of  isolation  began. 

During  these  periods  of  isolation,  the  organisms  in 
the  central  region  developed  much  more  rapidly  than  their 
respective  kindred  in  the  detached  region,  not  only  be- 
cause the  central  region  underwent  more  frequent  changes 
of  environment,  through  movements  of  the  earth's  crust, 
erosion,  etc.,  within  its  more  extensive  area,  but  because 
it  contained  a  greater  variety  of  environments,  and  hence 
developed  a  greater  variety  and  a  greater  number  of  or- 
ganisms. Therefore,  competition  for  food  and  other 
means  of  existence  was  very  sharp  in  the  central  region ; 
each  organism  was  compelled  to  use  all  its  faculties  to  se- 
cure a  livelihood,  and  all  but  the  more  perfectly  adapted 
organisms  perished.  Thus  the  more  lowly  organized 
forms  of  life  gradually  became  extinct,  and  were  succes- 
sively replaced  by  their  more  highly  organized  descend- 
ants. In  the  relatively  small  isolated  regions,  however, 
the  number  of  organisms  was  relatively  small,  and  the 
competition  for  food  was  not  sharp;  and  in  consequence 
the  descendants  of  the  lowly  organized  immigrants, 
though  changing  slightly,  developed  toward  a  higher  or- 
ganization with  extreme  slowness.  Thus,  upon  the  re- 
union of  an  isolated  region  with  the  central  land,  the 
migrants  from  the  latter  were  much  more  highly  organized 
than  the  inhabitants  of  the  newly  attached  region ;  and  if 
the  immigrants  were  numerous,  they  gradually  exterminated 
and  replaced  their  competitors  of  lower  organization. 


DISTRIBUTION    OF    LIFE.  349 

Australia  appears  to  have  had  but  one  such  union  with  the  central 
region,  and  that  at  a  very  early  period,  when  monotremes  and  mar- 
supials were  the  predominant  forms  of  mammalian  life.  South 
Africa  and  South  America  each  appear  to  have  had  a  succession  of 
such  unions  and  separations,  allowing  the  immigration  first,  of  low 
forms  only  (edentates,  lemurs,  etc.) ;  subsequently,  of  rodents  and 
small  carnivora;  and  lastly,  of  the  higher  forms  of  apes,  car- 
nivora,  and  hoofed-animals.  It  appears  that  North  America  and 
Euro-Asia  have  frequently  and  for  long  periods  been  more  closely 
united  in  the  arctic  regions  than  they  are  to-day,  and  at  times  when 
a  moderate  polar  climate  permitted  an  easy  interchange  of  organic 
forms ;  yet  there  probably  was  a  time  in  the  remote  past  when  the 
arctic  separation  was  more  complete  than  at  present,  and  when 
North  America  was  a  relatively  small,  isolated  region  of  the  great 
continent  of  Euro-Asia,  which  is  thus  probably  the  remote  source 
from  which  all  other  regions  were  supplied  with  their  higher  forms 
of  life.  At  that  time,  it  is  probable  the  Oriental  and  Euro-Asian 
regions  were  one — their  final  separation  dating  from  the  great  up- 
heaval of  the  Himalaya  Mountain  system. 

Of  the  distribution  of  marine  life,  and  the  laws 
which  govern  it,  but  very  little  is  known.  The  sea  being 
continuous,  and  the  water  below  a  comparatively  slight 
depth  having  an  almost  uniform  temperature,  it  is  not 
surprising  that  life-forms  in  various  regions  of  the  sea  do 
not  differ  so  greatly  as  in  the  various  regions  of  the  land. 
Nevertheless  there  are  differences,  the  reason's  for  which 
are  still  in  the  highest  degree  problematical ;  thus,  the 
king  crab  is  found  only  on' the  widely  separated  coasts  of 
Nova  Scotia,  Japan,  and  the  Malay  Archipelago.  Marine 
vegetable  life,  with  the  possible  exception  of  microscopic 
diatoms,  seems  to  be  confined  to  a  depth  of  less  than  100 
fathoms,  which  is  about  the  depth  to  which  the  luminous 
rays  of  the  sun  can  penetrate  the  water.  Marine  animal 
life,  however,  though  more  abundant  near  the  surface, 
exists  near  the  bottom  of  all  parts  of  the  oceans,  where, 
possibly  on  account  of  the  greater  food  supply,  it  is 
thought  to  be  more  abundant  than  at  intermediate  depths. 


CHAPTER  XXIV. 

MAN. 

Lord,  thou  deliveredst  unto  me  five  talents :  behold,  I  have  gained  beside  them 
five  talents  more.  His  lord  said  unto  him,  Well  done,  thou  good  and  faithful 
servant :  thou  hast  been  faithful  over  a  few  things,  I  will  make  thee  ruler  over 
many  things.— Matthew  xxv  :  20,  21. 

Man. — One  of  the  most  eminent  naturalists  and  stu- 
dents of  mankind  has  said:  "The  organized  world  pre- 
sents no  contrasts  and  resemblances  more  remarkable  than 
those  which  we  discover  on  comparing  mankind  with  the 
inferior  tribes.  That  creatures  should  exist  so  nearly  ap- 
proaching to  each  other  in  all  the  particulars  of  their 
physical  structure,  and  yet  differing  so  immeasurably  in 
their  endowments  and  capabilities,  would  be  a  fact  hard 
to  believe  if  it  were  not  manifest  to  our  observation.  In 
all  the  principles  of  his  internal  structure,  in  the  compo- 
sition and  functions  of  his  parts,  man  is  but  an  animal. 
The  lord  of  the  earth,  who  contemplates  the  eternal  order 
of  the  universe,  and  aspires  to  communion  with  its  invisi- 
ble Maker,  is  a  being  composed  of  the  same  materials  and 
framed  on  the  same  principles  as  the  creatures  which  he 
has  tamed  to  be  the  servile  instruments  of  his  will,  or 
slays  for  his  daily  food." 

The  fundamental  resemblance  of  man  to  all  animals 
without  exception,  from  the  most  highly  organized  man 
like  ape  down  to  the  single-celled  amoeba,  is  that  he  and 
they  alike  require  organic,  protein  food.  Man  resembles 
the  vertebrate  animals  much  more  closely  than  any  others, 
(350} 


MAN. 


351 


P.G. 


since  he  and  they  possess 
an  internal  bony  skeleton, 
built  upon  a  jointed  back- 
bone, containing  a  spinal 
nerve-cord,  which  leads  from 
a  principal  nerve-center,  or 
brain,  in  the  head.  Still 
closer  is  his  likeness  to  the 
mammals,  which,  like  man, 
nourish  their  young  from 
the  breast. 

While  possessing  a  close 
structural  resemblance  to  all 
mammals,  man  resembles  some 
kinds  much  more  closely  than 
others,  as  indicated  in  Fig.  146. 
The  resemblance  between  man 
and  the  man-like  apes  is  indeed 
much  closer  than  may  be  appre- 
ciated from  this  sketch,  for  even 
such  details  of  structure  as  de- 
termine the  difference  between 
the  hand  and  the  foot  of  man 
are  found  to  be  also  well  marked 
in  the  higher  members  of  the 
monkey  tribe,  which  therefore 
possess  true  hands  and  feet.  In 
fact,  the  extremities  of  the  man- 
like apes  differ  in  structure  less 
from  those  of  men  than  from 
those  of  the  lowest  monkeys,  or 
marmosets  (Fig.  147). 

Man  differs  most  widely 
from  all  the  lower  animals 
in  his  vastly  greater  mental 
capabilities.  The  organ  of 
the    mind — the    brain — is, 


352 


PHYSICAL    GEOGRAPHY. 


hand    GORILLA    £55j- 


haMd   CHIMPANZEE  foot  HAND  MARMOSET.  fo6t 

Fig.  147.— Hand  and  Foot  of  Man  and  Monkeys. 


on  the  average,  about  three  times  as  large  in  man  as  in 
those  animals  which  resemble  him  most  closely  in  struct- 
ure;— the  average  man  having  about  87  cubic  inches  of 
brain,  while  the  gorilla,  an  animal  about  twice  as  heavy 
as  man,  has  less  than  30  cubic  inches.  In  addition  to 
this,  the  surface  of  man's  brain  is  furrowed  by  a  vastly 
more  complicated  system  of  fissures,  or  sulci,  and  inter- 
vening folds,  or  convolutions.  The  surface  area  of  man's 
brain  is  thus  greatly  enlarged,  and  mental  power  is  sup- 
posed to  depend  to  a  great  extent  upon  the  surface  area 
of  the  brain  (Fig.   148). 

Though  the  brain  is  larger  and  mental  power  immeasurably- 
greater  in  man  than  in  the  higher  beasts,  the  difference  in  the 
structure  of  the  brain  is  relatively  slight.  Prof.  Huxley,  one  of  the 
greatest  anatomists  of  the  present  age,  has  carefully  compared  the 
brain  of  man  with  that  of  various  members  of  the  monkey  tribe, 
and  has  found  that  "  so  far  as  cerebral  (brain)  structure  goes,  man 
differs  less  from  the  chimpanzee  or  orang-outang  than  these  do  from 


MAN. 


353 


the  (lower)  monkeys,"  while  "the  (structural)  difference  between 
the  brains  of  the  chimpanzee  and  of  man  is  almost  insignificant, 
when  compared  with  that  between  the  chimpanzee  brain  and  that  of 
the  lemur  (the  peculiar  half-monkey  of  Madagascar).  The  argu- 
ment," Huxley  continues,  "that  because  there  is  an  immense  differ- 
ence between  a  man's  intelligence  and  an  ape's,  therefore  there 
must  be  an  equally  immense  difference  between  their  brains,  appears 
to  me  about  as  well  based  as  the  reasoning  by  which  one  would 
endeavor  to  prove,  that  because  there  is  a  'great  gulf  between  a 
watch  that  keeps  accurate  time  and  another  that  will  not  go  at  all, 
there  is  therefore  a  great  structural  hiatus  (difference)  between  the 


LEMUR  ORANG  MAN 

Fisf.  148.— Brain  of  Man  and  Monkeys. 


two  watches.  A  hair  in  the  balance  wheel,  a  little  rust  on  a  pinion, 
a  bend  in  a  tooth  of  the  escapement,  a  something  so  slight  that  only 
the  practiced  eye  of  the  watchmaker  can  discover  it,  may  be  the 
source  of  all  the  difference.  And  believing,  as  I  do  (with  Cuvier), 
that  the  possession  of  articulate  speech  is  the  grand  distinctive 
character  of  man,  I  find  it  easy  to  comprehend  that  some  equally 
inconspicuous  structural  difference  may  have  been  the  primary 
cause  of  the  immeasurable  and  practically  infinite  divergence  of  the 
human  from  the  ape  (family)." 

The  fact  that,  physically  considered,  man  resembles 
the  higher  beasts  as  closely  as  these  resemble  the  lower 


354  PHYSICAL    GEOGRAPHY. 

animals,  in  connection  with  the  fact,  now  generally  ad- 
mitted, that  the  higher  and  lower  animals  are  but  differ- 
ently modified  descendants  of  a  common  kind  of  ancestors, 
leads  to  the  inference,  that  man  himself  is  a  still  differently 
modified  descendant  of  the  same  remote  ancestors.  The 
difference  in  the  degree  of  mental  power,  however,  be- 
tween civilized  man  and  even  the  highest  beast,  is  appar- 
ently so  much  greater  than  that  between  the  highest  and 
even  the  very  lowest  animal,  that  it  is  hard  to  conceive  of 
any  natural  process,  by  which  such  an  almost  infinite  diver- 
gence could  have  taken  place  in  organisms  descended, 
however  remotely,   from  similar  ancestors. 

Whatever  may  have  been  the  origin  of  man,  and 
whatever  may  be  his  true  relationship  to  the  higher  ani- 
mals which  he  resembles  so  closely  in  structure,  we  know 
that  he  has  been  an  inhabitant  of  the  earth  for  a  time 
very  much  greater  than  the  4,000  or  5,000  years  of  which 
history  or  tradition  preserves  a  record.  This  is  known 
from  the  occurrence  of  fossils  of  man — human  bones,  and 
implements  made  by  man — associated  in  deposits  with  the 
fossils  of  extinct  animals  of  the  late  tertiary  or  early  qua- 
ternary era.  This,  though  quite  recent  when  compared 
with  the  lapse  of  geological  time,  indicates  that  man,  as  a 
tool-  or  implement-making  animal,  inhabited  the  earth 
tens,  or  possibly  hundreds,  of.  thousands  of  years  ago, 
while  relics  of  man,  found  in  deposits  of  intermediate  ages, 
indicate  that  he  has  inhabited  the  earth  continuously  dur- 
ing this  long  but  indefinite  period. 

We  know  that  man  has  changed  but  little  in  his 
structure  and  manner  of  thought  during  the  period  cov- 
ered by  history  and  tradition,  but  that  his  general  knowl- 
edge and  intelligence  have  increased  greatly  during  this 
time.  Although,  at  the  dawn  of  history  or  tradition, 
man  in  ancient  Egypt  dwelt  under  an  organized  govern- 


man.  355 

merit,  knew  the  use  of  the  more  useful  metals,  practiced 
the  art  of  agriculture,  had  sufficient  knowledge  of  me- 
chanics to  build  monuments  which  have  endured  until  the 
present,  and  was  thus  vastly  more  civilized  than  many 
savage  tribes  are  to-day ;  still,  with  all  these  attainments, 
his  civilization  was  very  greatly  inferior  to  that  of  the 
present  time.  In  following  the  history  of  mankind  from 
that  day  to  this,  one  may  note  the  more  or  less  gradual 
increase  of  knowledge,  its  broader  diffusion  among  the 
masses,  and  the  consequent  slow,  but  on  the  whole  con- 
tinuous, progress  in  general  intelligence  and  civilization. 
The  same  kind  of  progress  in  the  intelligence  of  pre- 
historic man  may  be  dimly  traced,  by  comparing  his 
implements  and  other  indications  of  his  work  and  habits, 
as  they  are  occasionally  found  in  deposits  of  successively 
older  date.  As  the  age  of  the  deposit  increases,  the  im- 
plements become  less  various  in  shape,  and  simpler  and 
ruder  in  construction ;  while  the  associated  remains,  when 
they  afford  any  indication  of  the  manner  of  life  of  pre- 
historic man,  indicate  that  this  was  simpler  and  ruder  in 
proportion  as  the  deposit  is  older. 

Thus,  in  going  backward  from  the  beginning  of  historic  times  in 
various  parts  of  Europe,  we  find  that  the  more  modern  prehistoric 
races  of  portions  of  that  region  made  implements  of  iron  and 
bronze,  as  well  as  of  stone ;  had  domesticated  such  animals  as  the 
ox,  dog,  sheep,  and  goat;  lived  together  in  settlements,  and  culti- 
vated wheat,  and  the  same  kind  of  barley  found  wrapped  with  old 
Egyptian  mummies.  In  earlier  deposits,  only  implements  of  bronze 
and  stone  are  found,  while  the  associated  bones  of  the  dog,  horse,  and 
ox,  and  other  remains  indicate  a  pastoral  rather  than  an  agricultural 
life.  From  still  older  deposits  come  only  implements  of  stone  or 
bone,  while  the  dog  seems  to  be  the  only  domestic  animal.  These 
stone  implements,  however,  are  neatly  and  symmetrically  shaped, 
and  have  generally  been  ground  down  to  a  smooth  and  often  polished 
surface,  indicating  a  degree  of  intelligence  in  the  makers  decidedly 
greater  than  that  found  in  the  rudest  savages.     The  oldest  of  all 


356  PHYSICAL    GEOGRAPHY. 

implements  of  prehistoric  man  are  those  found  associated  with 
fossils  of  animals  now  extinct.  These  earliest  implements  are  also 
of  stone,  but  are  much  more  rudely  made  than  the  later  ones,  being 
simply  flakes  of  flint,  or  other  hard  stone,  roughly  chipped  into  un- 
symmetrical  shapes  to  obtain  a  cutting  edge  or  a  sharp  point.  Such 
implements  are  to-day  made  and  used  by  the  lowest  and  most 
ignorant  tribes  of  savages,  but  are  found  in  more  or  less  deeply 
buried  deposits  in  all  parts  of  the  world,  as  the  most  ancient 
vestiges  of  man. 

Such  facts  as  these  are  held  to  indicate  that  all 
men — the  most  cultivated  races  as  well  as  the  rudest — 
have  descended  from  more  or  less  remote  ancestors  who 
were  as  ignorant,  and  as  low  in  the  scale  of  intelligence 
and  civilization,  as  the  lowest  savages  of  whom  we  have 
any  knowledge.  During  the  vast  period  of  time  which 
has  elapsed  since  all  mankind  was  at  this  low  state,  dif- 
ferent portions  of  the  human  family  have  developed  their 
mental  powers  at  different  rates,  resulting  in  the  various 
degrees  of  intelligence  and  civilization  found  among  the 
people  now  inhabiting  different  regions  of  the  globe. 

It  is  to  be  remarked  that  though  the  indications  of  this  progres- 
sive development  of  the  human  intelligence  are  so  strong  and  nu- 
merous as  to  render  the  fact  practically  certain,  still  the  most  ancient 
traces  of  man  yet  found  indicate  a  being  no  less  distinctly  human 
than  are  the  lowest  savages  of  Borneo  and  Tierra  del  Fuego  to-day. 
But  the  gap  separating  the  intelligence  of  a  naked  savage  of  the 
Borneo  forest  from  that  of  an  Emerson,  a  Spencer,  a  Gladstone,  or 
a  Bismarck,  is  very  great,  and  the  daily  accumulating  evidence  that 
the  latter  has  developed  by  gradual  modification  from  the  former 
has  suggested  to  some  that  the  low  intelligence  of  the  savage  may 
itself  have  developed,  by  a  similar  process,  during  the  long  ages  of 
the  remote  past,  from  a  still  lower  state,  in  which  it  was  similar  to, 
or  identical  with,  what  we  call  instinct  in  animals. 

Certain  superficial  differences  in  physical  features 
are  found  to  distinguish  men  who  for  long  periods  have 
lived  in  separated  regions.  Because  these  differences  are 
superficial,   they  are  often  quits  conspicuous;    but  when 


man.  357 

the  deeper  seated  and  more  essential  structural  features 
are  compared,  they  are  found  to  be  so  wonderfully  sim- 
ilar in  men  from  every  region,  as  to  warrant  the  belief  that 
all  mankind  is  descended  from  a  single  race. 

An  Englishman  can  generally  be  distinguished  from  a  Dutch- 
man by  indefinable  peculiarities  of  physical  feature;  but  it  is  well 
known  that  the  ancestors  of  these  peoples  formed  a  single  race, 
and  lived  together  in  the  region  between  Denmark  and  Belgium. 
About  the  year  500,  a  portion  of  this  people  went  over  and  con- 
quered Britain,  where  they  settled  and  continued  to  dwell.  These 
emigrants  did  not  differ  materially  in  feature  from  their  former 
neighbors  who  remained  at  home,  but  their  descendants  were  in 
great  part  isolated  from  their  kindred  on  the  main-land,  and  each 
portion,  by  adaptation  to  its  special  environment,  gradually  devel- 
oped the  peculiar  features  which  characterize  either  people  to-day. 
The  fact  that  the  slight  but  plainly  perceptible  race  differences  be- 
tween the  English  and  the  Dutch  have  thus  developed  during  1,400 
years  of  imperfect  isolation  of  the  two  peoples,  in  two  regions  so 
closely  adjacent  as  to  have  nearly  the  same  climate  and  general 
surroundings,  is  held  to  afford  conclusive  evidence  that  the  greatest 
differences  between  the  most  divergent  races  of  men,  may  be  ac- 
counted for  by  the  operation  of  the  same  processes,  during  the 
vastly  longer  period  of  man's  occupancy  of  the  earth,  and  on  de- 
scendants of  an  originally  similar  people,  who  became  completely 
isolated  from  each  other,  in  regions  so  widely  separated  as  to  differ 
markedly  in  climate  and  other  conditions  of  environment. 

Resemblances  in  language  and  customs  are  often 
found  in  races  which  now  occupy  widely  separated  regions 
and  differ  markedly  from  each  other  in  physical  feature. 
While  such  resemblances  can  not  be  found  between  all 
languages,  they  are  thought,  when  they  do  occur,  to 
afford  direct  evidence  that  these  particular  languages  are 
but  more  or  less  divergent  variations  from  a  single  prim- 
itive tongue,  and  that  the  races  using  them  are  descend- 
ants of  the  single  race  that  used  the  primitive  language. 

Indeed,  language  is  thought  to  afford  the  best  available  means 
for  tracing  the  connection  between  various  races.    If  the  resemblance 


358  PHYSICAL    GEOGRAPHY. 

is  strong,  involving  whole  sentences  or  very  many  words,  the  people 
are  supposed  to  have  been  separated  for  a  relatively  short  time.  If 
the  resemblance  is  only  in  an  occasional  word,  the  separation  of  the 
languages  and  the  people  using  them,  from  the  parent  stock,  is  thought 
to  be  of  very  ancient  date.  If  no  resemblance  at  all  can  be  traced 
between  languages,  the  separation  of  the  people  using  them  from 
a  common  ancestral  race  is  thought  to  have  occurred  at  an  exceed- 
ingly remote  period,  possibly  before  the  race  had  acquired  a  common 
language. 

The  inevitable  tendency  in  any  people  to  change, 
which  accompanies  broad  distribution  and  the  consequent 
variety  in  environment,  implies  that  the  race  from  which 
the  whole  human  family  is  thought  to  have  descended, 
originally  occupied  a  region  of  rather  limited  extent,  and 
that  the  world  was  peopled  by  the  descendants  of  various 
portions  of  this  race,  who  gradually  wandered  from  their 
ancestral  home  in  different  directions. 

While  there  is  thus  some  reason  for  supposing  that  man  overran 
the  earth  at  an  immensely  remote  period,  from  some  rather  small 
central  region,  it  is  utterly  impossible,  in  the  present  state  of  knowl- 
edge, to  locate  this  region  with  any  degree  of  accuracy.  The  fact 
that  the  regions  inhabited  by  the  three  most  widely  divergent  types 
of  mankind  at  the  present  time,  approach  each  other  most  closely 
in  southern  and  south-western  Asia,  is  held  by  many  to  indicate  that 
the  ancestral  home  of  man  was  in  that  region ;  and  the  fact  that  all 
of  the  higher  animals  seem  to  have  had  their  earliest  development 
in  the  great  land  mass  of  the  northern  hemisphere,  may  be  said  to 
favor  the  view  that  man,  the  highest  of  all  organisms,  was  not  an 
exception  to  this  rule. 

Classification  of  Mankind. — Varieties  of  men  are 
usually  distinguished  by  differences  in  the  character  of  the 
hair,  formation  of  the  language,  color  of  the  skin,  and 
shape  of  the  skull.  The  formation  of  the  hair,  and  to  a 
lesser  extent  the  color  of  the  skin,  seem  to  be  more 
strictly  hereditary  than  the  form  of  the  skull;  and  from 
more  or  less  conspicuous  differences  in  these  features,  all 
mankind  may  be  divided  into  three  broad  classes, or  types: 


MAN. 


359 


m. 


Woolly-haired.  Straight-haired.  Wavy-haired. 

Fig.  149.— The  Three  Types  of  Mankind. 

(1)  the  woolly-haired  and  brown-skinned  type;  (2)  the 
straight-haired  and  yellowish-skinned  type ;  and  (3)  the 
wavy-haired  and  whitish-skinned  type. 

These  types  differ  from  each  other  entirely  in  the  formation  of 
the  languages  used,  and  each  type  includes  several  groups,  or  races, 
which  resemble  each  other  in  the  more  general  type-characteristics, 
but  generally  differ  widely  in  language  and  in  minor  details  of  feat- 
ure, while  each  race  is  subdivided  chiefly  by  minor  differences  of 
language  into  smaller  groups,  or  tribes.  The  different  tribes,  races, 
and  types,  however,  graduate  insensibly  into  each  other  from  long- 
continued  cross  marriages  between  different  peoples,  so  that  it  is 
often  impossible  to  draw  hard  and  fast  lines  between  them. 

The  woolly-haired  type  is  characterized  by  its  woolly 
or  kinkled  hair,  and  by  the  brownish  color  of  the  skin, 
which  ranges  from  almost  black  to  a  light  brownish  tint. 
The  peculiar  character  of  the  hair  results  from  the  fact 
that  each  hair,  when  duly  magnified,  is  found  to  be  flat, 
or  tape-like.  As  a  rule,  the  head  in  this  type  is  very  long 
from  front  to  back  in  proportion  to  its  width,  and  the 
jaws  generally  project  forward,  giving  the  profile  of  the 
face  a  backward  slant  from  the  mouth  to  the  low,  reced- 
ing forehead.  This  peculiarity  is  stronger  in  some  tribes 
than  in  others ;  it  is  still  stronger  in  the  monkey  tribe, 
and  is  most  strongly  marked   in  the  quadrupeds.     The 


360  PHYSICAL    GEOGRAPHY. 

mental  development  of  this  type  as  a  whole  is  lower  than 
that  of  the  other  types.  No  native  woolly-haired  race 
has  ever  had  a  written  history.  All  races  of  this  type 
are  native  in  the  southern  hemisphere,  which  is  thus  char- 
acterized in  its  human,  as  well  as  in  its  animal  inhabitants, 
by  a  relatively  low  state  of  development. 

There  are  four  principal  races  of  the  woolly-haired  type.  (1)  The 
Papuan  race  inhabits  the  islands  from  New  Guinea  east  to  the 
Fijis,  the  mountainous  interior  of  the  Malay  peninsula,  Borneo,  the 
Philippines,  and  several  islands  of  the  Pacific.  This  race  has  but 
lately  become  extinct  in  Tasmania.  This  race  is  the  lowest  of  the 
type,  is  nearly  black,  with  thick,  protruding  lips  and  kinkled  hair, 
growing  in  separate  tufts  over  the  head.  (2)  The  Hottentots  are 
now  confined  to  the  southern  part  of  Africa  (Namaqua  Land  and 
the  interior  of  Cape  Colony),  and  are  rapidly  approaching  extinction. 
Though  the  skin  is  a  yellowish-brown,  this  race  resembles  the 
Papuan  in  having  a  very  flat  face,  thick,  protruding  lips,  and  hair 
growing  in  separate  tufts.  (3)  The  Kaffres  inhabit  the  rest  of  South 
Africa.  To  this  race  belong  the  Zulu,  Zambezi,  and  Mozambique 
tribes  on  the  east,  the  great  Bechuana  peoples  in  the  interior,  and 
the  Herrero  and  Kongo  tribes  on  the  west  coast.  Unlike  the  two 
preceding  races,  the  woolly  hair  of  the  Kaffre  race  grows  as  a  con- 
tinuous fleece  over  the  head.  The  race  has  a  high  forehead,  a 
prominent  nose,  and  but  slightly  protruding  lips.  (4)  The  Negro 
race  inhabits  the  Soudan,  and  the  southern  part  of  the  Sahara  from 
the  upper  course  of  the  Nile  to  the  Atlantic.  The  skin  is  very  dark 
brown,  and  velvety  to  the  touch;  the  woolly  hair,  like  the  Kaffre's, 
grows  as  a  fleece,  but  the  forehead  is  flatter  and  lower,  the  nose 
broad  and  thick,  and  the  lips  large  and  protruding. 

The  straight-haired  type  of  mankind  is  characterized 
by  its  coarse,  straight  black  hair,  each  hair  being  cylin- 
drical,—  that  is,  having  a  circular  section.  The  color  of 
the  skin  varies  from  brown  through  yellow  to  a  reddish, 
but  generally  a  yellowish  tone  is  present.  Many  of  the 
races  of  this  type  are  round  headed,  the  length  and  width 
of  the  skull  when  seen  from  above  being  nearly  equal. 
The  forehead  is  generally  less  receding,  the  jaws  less  pro- 


MAN.  361 

tuberant,  and  the  mental  development  is  higher  as  a  rule 
than  in  the  woolly-haired  type.  One  race,  however — the 
Australian — is  classed  with  this  type  on  account  of  its 
straight,  coarse  hair;  but  it  has  the  dark  color,  slanting 
face,  and  protruding  lips  of  the  woolly-haired  type,  and  is 
considered  to  represent  one  of  the  lowest  states,  if  not  the 
lowest  state,   of  mental  development  in  living  man. 

There  are  five  races  of  this  type.  (1)  The  Australian  race  is 
confined  to  the  main-land  of  Australia.  The  mental  and  physical 
development  of  this  race  is  very  low,  the  bones  being  remark- 
ably weak  and  delicate  in  structure.  It  is  a  significant  fact  that 
Australia,  which  is  thus  occupied  by  probably  the  least  highly  or- 
ganized race  of  men,  should  also  be  strongly  characterized  by  the 
lowest  of  the  mammals — the  monotremes  and  the  marsupials.  (2) 
The  Malay  race,  though  not  numerous,  is  very  widely  distributed, 
embracing  the  bulk  of  the  people  in  the  Malay  peninsula,  the 
Malay  Archipelago,  and  most  of  the  islands  of  the  Pacific  and 
Indian  oceans,  from  the  Hawaiian  Islands  on  the  east  to  New 
Zealand  and  Madagascar  on  the  west.  (3)  The  Mongolian  forms  one 
of  the  most  numerous  races  on  the  earth,  embracing  in  its  many 
tribes  almost  all  the  inhabitants  of  Asia  from  Okhotsk  Sea  to  the 
Bay  of  Bengal,  and  westward  north  of  the  Himalaya  Mountains  into 
eastern  Europe,  while  the  Lapps  and  Finns  of  Scandinavia,  the 
Volga  Finns  of  central  Russia,  the  Magyars  of  Hungary,  and  the 
Turks  of  the  Balkan  peninsula  and  Asia  Minor  are  isolated  tribes  of 
this  race.  The  many  languages  spoken  by  different  peoples  of  this 
race  may  be  divided  into  two  groups,  which  are  very  remotely  con- 
nected. The  skin  of  the  race  is  always  yellowish  in  tone,  but  varies 
in  different  tribes  from  a  dark  brownish-yellow  to  a  light  greenish- 
yellow.  The  face  is  generally  round,  with  prominent  cheek  bones, 
while  the  eye-openings  are  narrow,  and  generally  slant  downward 
toward  the  nose.  (4)  The  Esquimos  inhabit  Kamchatka  and  the 
north-east  extremity  of  Asia,  the  Arctic  Archipelago,  and  a  narrow 
strip  of  the  Arctic  American  coast  from  Alaska  peninsula  to  New- 
foundland. The  Esquimos  are  short,  of  stout  build,  with  the  round 
face  and  slanting  eyes  of  the  Mongolians,  and  a  brownish  skin, 
toned  with  yellow  or  yellowish-red.  (5)  The  Americans,  or  Red-skins, 
occupy  both  North  and  South  America.  Many  extremely  different 
languages  prevail  in  this  wide  extent  of  territory,  yet  all  may  be 


362  PHYSICAL   GEOGRAPHY. 

referred  to  a  single  primitive  tongue.  This  race  seems  most  closely 
related  to  the  Esquimo  and  Mongolian.  It  is  characterized  by  a 
medium-shaped  head  neither  long  nor  round ;  straight,  black  hair  ; 
broad,  low  forehead ;  prominent  nose  and  cheek  bones,  thin  lips, 
and  a  brownish  skin,  strongly  tinged  with  red  or  reddish-yellow. 

The  wavy-haired  type  of  the  human  family  is  distin- 
guished by  hair  much  softer  than  that  of  either  of  the 
other  types.  It  is  neither  lank  nor  kinkled,  but  usually  is 
inclined  to  be  wavy  whenever  allowed  to  grow  long.  The 
section  of  each  hair  is  elliptical  in  shape.  In  this  type  the 
beard  grows  much  more  freely  and  thickly  than  in  either 
of  the  others.  The  face  is  oval  in  shape,  the  forehead 
high  and  prominent,  and  the  jaws  do  not  protrude  ;  hence, 
the  general  profile  of  the  face  is  nearly  vertical.  The 
color  of  the  skin  varies,  as  in  the  other  types,  but  in  the 
vast  majority  of  cases  is  so  much  lighter  as  to  be  called 
white  in  comparison ;  but  it  is  usually  tinged  with  pink, 
and  in  some  instances  is  a  dark  brown.  The  type  includes 
races  which  vary  widely  in  mental  development,  but  as  a 
whole  it  may  be  said  to  have  reached  a  decidedly  more 
advanced  state  than  any  other  type. 

There  are  three  races  of  the  wavy-haired  type.  (1)  The  Dravidian 
race  is  confined  to  Ceylon  and  the  plateau  of  Deccan.  This  race 
is  exceedingly  difficult  to  classify.  It  has  the  wavy  hair  and  strong 
beard  of  this  type,  and  the  language  bears  a  resemblance  to  that  of 
some  of  the  Mediterranean  tribes,  but  the  skin  varies  from  a  light 
yellowish-brown  to  a  very  dark  brown.  The  face  is  oval,  the  fore- 
head high,  nose  narrow  and  prominent,  but  the  lips  are  slightly 
protruding.  (2)  The  Nubian  race  is  also  hard  to  classify.  It  in- 
habits Nubia,  Kordofan,  and  Dongola,  and  various  tribes  of  the 
northern  Soudan  seem  to  carry  it  westward  nearly  to  the  Atlantic. 
The  hair  is  brown  or  black,  and  wavy — not  woolly — and  the  lan- 
guage has  no  resemblance  to  that  of  any  negro  tribe.  The  beard  is 
well  developed,  the  face  oval,  forehead  high,  and  nose  prominent, 
but  the  skin  is  a  dark  yellowish-  or  reddish-brown.  (3)  The  Cau- 
casian or  Mediterranean  race  is  the  most  highly  developed  of  the 


MAN.  363 

races.  It  formerly  inhabited  a  region  extending  from  the  Bay  of 
Bengal  west  to  the  Atlantic,  embracing  south-west  Asia,  northern 
Africa,  and  nearly  the  whole  of  Europe ;  but  during  the  last  500 
years,  representatives  of  this  race  have  spread  over  nearly  the  whole 
globe.  This  race  is  the  most  numerous  on  earth,  and,  with  the 
Mongolian  race,  is  the  only  portion  of  mankind  possessing  a  writ- 
ten history.  The  hundreds  of  different  languages  and  dialects  of 
the  modern  descendants  of  the  Mediterranean  race  may  be  traced 
to  four  distinct  primitive  languages,  and  upon  this  the  classification 
of  the  race  into  four  main  branches  is  chiefly  based ;  viz.,  (a)  The 
Basques,  who  formerly  occupied  the  whole  of  south-western  Europe, 
but  are  now  confined  to  a  narrow  region  on  the  northern  coast  of 
Spain,  (b)  The  Caucasians,  confined  to  a  small  territory  between 
the  Black  and  Caspian  seas  about  the  Caucasus  Mountains,  (c)  The 
Semitic  branch,  including  in  one  group  the  Berbers  of  the  Sahara 
west  to  the  Atlantic,  the  Ethiopian,  Galla,  Somali,  and  other  tribes 
on  the  north-east  coast  to  the  equator ;  and  in  another  group  the 
Jews,  Syrians,  ancient  Chaldeans,  and  Arabs,  a  tribe  of  the  latter 
forming  the  inhabitants  of  Abyssinia,  (d)  The  Indo-Germanic  or 
Aryan  branch,  which  includes  the  ancestors  of  the  Hindoos  and  Per- 
sians ;  the  Graeco-Romans,  ancestors  of  the  Greeks,  Albanians,  and 
Italians;  the  Celts,  ancestors  of  the  ancient  Gauls,  the  Irish,  and 
Welsh  ;  the  Slavonians,  ancestors  of  the  Russians  and  Bulgarians ; 
and  the  ancient  Germans,  or  ancestors  of  the  modern  Germans, 
Dutch,  Scandinavians,  Anglo-Saxons  or  Englishmen,  and  of  a  vast 
majority  of  the  present  inhabitants  of  the  United  States. 

The  present  population  of  the  world  is  estimated  at 
about  1,450  millions  of  individuals.  About  1,200  millions, 
nearly  83%  of  mankind,  are  included  in  the  Mediterranean 
and  Mongolian  races,  and  both  of  these  races  seem  to  be 
increasing  in  number  and  in  intelligence,  the  increase  and 
progress  of  the  Mediterranean  race  being  especially  rapid. 
The  other  ten  principal  races  of  mankind  are  estimated 
to  include  at  present  less  than  17%  of  the  population  of 
the  world,  and  these  races  seem  to  be  on  the  whole  slowly 
decreasing  in  number  and  approaching  extinction,  as  a 
direct  or  indirect  result  of  the  influence  of  the  more  intel- 
ligent and   civilized   Mediterranean  race.     The  estimated 


364  PHYSICAL   GEOGRAPHY. 

number  of  individuals  in  each  of  the  twelve  principal  races 
of  mankind  is  given  below : 


Mediterranean,   .     .  625,000,000 

Mongolian,     .     .     .  575,000,000 

Negro, 130,000,000 

Dravidian,      .     .     .  34,000,000 

Malay, 30,000,000 

Kaffre, 20,000,000 

American  Indian,    .  12,000,000 

Nubian, 10,000,000 


Papuan, 2,000,000 

Australian,  ....  100,000 

Hottentot,    ....  100,000 

Esquimo,     ....  100,000 

Half-breeds  of  the  {      .    ' „ 

y  11,700,000 

various  races        J  "     ' 


Total,  1,450,000,000 


Man  in  the  rudest  state  in  which  he  now  exists,  is 
the  most  dominant  creature  that  has  ever  appeared  upon 
the  earth.  He  forms  the  only  highly  organized  species 
that  has  spread  over  the  entire  land  surface  of  the  globe. 
All  other  organisms  have  yielded  before  him.  All  savage 
men  without  exception  seem  to  possess  an  articulate  lan- 
guage, a  knowledge  of  the  art  of  making  fire  and  of  some 
of  its  uses,  and  the  ability  to  make  and  use  various  rude 
weapons,  tools,  and  traps  with  which  to  defend  themselves 
and  obtain  food.  Such  inventions,  by  which  the  rudest 
savage  achieves  his  pre-eminence  among  organisms,  are 
the  direct  results  of  the  development  of  his  powers  of 
observation,   memory,   curiosity,  imagination,  and  reason. 

The  reasons  why  certain  tribes,  and  not  others,  have 
risen  in  the  scale  of  civilization  from  this  rude  state  can 
not  be  fully  given.  Progress  seems  to  depend  upon  com- 
binations of  favorable  conditions  far  too  complex  to  be 
followed  out.  The  remarkable  fact,  however,  has  fre- 
quently been  observed  that  all  high  civilization  has  devel- 
oped in  the  north  temperate  zone,  and  that  the  native 
races  of  this  zone,  when  first  visited  by  civilized  men,  had 
arrived  at  a  higher  state  of  civilization  than  the  native 
tribes  of  the  torrid  and  frigid  zones.  It  would  seem, 
therefore,  that  an  extensive  land  area,   combined  with  a 


MAN.  365 

temperate  climate,  is  a  physical  condition  favorable  to  the 
development  of  intelligence  and  civilization. 

Reflection  suggests  a  possible  explanation  for  this.  Development 
is  the  result  of  mental  activity,  and  hence  would  generally  be 
greatest  in  a  region  that  afforded  the  greatest  incentive  to  mental 
action.  An  extensive  region  possesses  a  greater  variety  of  environ- 
ments than  a  contracted  region,  and  hence  would  develop  a  greater 
number  of  different  tribes  of  men.  The  natural  competition  of 
these  tribes  for  mastery,  would  form  a  constant  incentive  to  greater 
mental  activity  in  the  larger  region.  In  extensive  temperate  regions, 
an  additional  incentive  to  mental  action  is  afforded  by  the  effect  of 
the  climate  upon  vegetable  and  animal  life,  and  hence  upon  man's 
food  supply.  The  regular  alternation  in  such  regions  of  a  long, 
warm  summer,  when  food  is  plenty,  with  a  long,  cold  winter,  when 
food  is  very  scarce,  is  in  marked  contrast  to  the  constant  summer 
of  the  torrid  zone,  which  affords  a  perpetual  abundance  of  food  in 
the  moist  regions,  but  causes  a  perpetual  dearth  of  food  in  the  dry, 
desert  regions ;  while  in  the  frigid  zones  there  is  a  perpetual  dearth 
of  food  because  of  the  long,  cold  winters.  Hence,  the  climate  of 
the  temperate  zones  is  peculiar  in  affording  a  constant  incentive  to 
collect  and  cure  a  store  of  food  during  the  summer  upon  which  to 
draw  during  the  following  winter.  The  gradual  development  of  fore- 
sight and  ingenuity  involved  in  such  a  collection  and  preservation 
of  food  for  future  use,  would  of  itself  raise  a  tribe  in  the  scale  of 
civilization,  and  such  mental  development  would  lead  to  further 
progress  in  other  directions. 

Domestic  Plants  and  Animals. — The  great  mass  of 
mankind  to-day — all,  indeed,  but  the  rudest  tribes — de- 
pend chiefly  for  food  and  clothing  upon  domestic  plants  and 
animals.  These  form  the  portion  of  the  organic  world 
which  man  has  subjugated.  It  is  a  remarkably  small  por- 
tion when  compared  with  the  world's  flora  and  fauna. 
Among  more  than  a  million  species  of  plants  and  animals, 
the  cultivated  plants  form  only  about  300  species,  and  the 
domestic  animals  only  about  200  species.  By  far  the 
most  important  of  these  plants  and  animals  were  reduced 
to  a  domestic  state  in  prehistoric  times ,  and  can  not  now  be 


366  PHYSICAL    GEOGRAPHY. 

found  in  a  truly  wild  state.  That  is  to  say,  they  have  been 
under  the  peculiar  environment  caused  by  man's  care,  long 
enough  to  have  varied  in  structure  to  such  an  extent,  that 
the  respective  wild  species,  from  which  they  originally 
sprung,   can  no  longer  be  identified  with  certainty. 

The  food  plants,  wheat,  oats,  barley,  rye,  rice,  sugar-cane,  tea; 
many  garden  vegetables,  as  the  turnip,  onion,  cabbage,  cucumber, 
watermelon,  bean,  and  pea ;  several  fruits,  as  the  European  grape, 
mango,  apricot,  almond,  peach,  pear,  apple,  quince,  pomegranate, 
olive,  fig,  date,  and  banana;  the  fiber  plants,  cotton,  flax,  and 
hemp ;  and  the  common  domestic  animals,  the  horse,  ass,  camel,  and 
sheep,  goats,  cattle,  and  chickens,  had  been  cultivated  and  domesti- 
cated by  various  races  of  Euro-Asia  before  the  dawn  of  their  re- 
spective histories,  and  in  very  early  historic  time  coffee  was 
cultivated.  The  native  races  of  America,  upon  its  discovery,  though 
they  knew  nothing  of  the  domestic  plants  and  animals  mentioned 
above,  had  domestic  plants  and  animals  of  their  own  that  were 
equally  strange  to  the  Europeans,  such  as  the  sweet-potato,  the 
potato,  maize  or  corn,  the  pumpkin,  squash,  tomato,  mate  or  Para- 
guay tea,  coca,  cacao,  the  aloe,  guava,  pine-apple,  peanut,  tobacco, 
red  pepper,  and  sea-island  cotton,  and  the  turkey,  rabbit,  guinea- 
pig,  and  llama.  It  is  thus  seen  that  the  very  plants  and  animals 
which  America  now  supplies  in  such  great  quantities  to  the  markets 
of  the  world,  such  as  wheat,  rice,  sugar-cane,  coffee,  hemp,  the 
common  kind  of  cotton,  and  horses,  cattle,  sheep,  swine,  and 
chickens,  are  all  of  foreign  origin,  and  were  introduced  by  man 
since  the  discovery  of  the  continent  only  400  years  ago ;  while  the 
potato  and  Indian  corn,  which  are  now  largely  raised  in  parts  of 
Europe,  were  introduced  from  America,  where  they  had  previously 
been  sparingly  cultivated  by  the  natives. 

Man's  achievements  over  inorganic  nature  have  in 
general  been  of  much  more  recent  date  than  those  over 
the  organic  world.  In  fact,  the  progress  of  civilization 
during  historic  time  is  almost  exclusively  the  result  of 
discoveries  and  inventions  by  which  inorganic  substances 
can  be  applied  to  the  uses  of  man,  and  this  is  specially 
true  of  the  present  age. 


MAi>.  367 

Metals. — Among  the  inorganic  substances  whose  use 
by  man  indicates  his  progress  in  civilization,  are  the 
metals.  Few  of  these  occur  in  a  pure  state  in  nature. 
Most  of  them  are  found  as  stony  substances,  or  ores,  in 
which  the  metal  occurs  only  as  a  chemical  ingredient,  and 
from  which  it  is  obtained  only  by  more  or  less  intricate 
artificial  processes.  Some  metals  are  found  in  the  metallic 
state,  but  almost  always  alloyed,  or  mixed,  with  other 
metals,  and  their  separation  is  generally  difficult.  But 
even  after  the  pure  metal  is  obtained,  more  or  less  difficult 
artificial  processes  are  required  to  apply  it  to  the  various 
purposes  of  man.  It  is  chiefly  through  an  increasing 
knowledge  of  the  laws  of  nature,  by  which  the  metals 
and  other  inorganic  substances  can  be  more  easily  ob- 
tained and  applied,  that  modern  civilization  is  advancing. 

The  eight  metals  in  most  common  use  to-day — iron,  copper,  tin, 
zinc,  lead,  gold,  silver,  and  mercury — seem  to  have  been  known  in 
the  earliest  historic  time ;  but  as  geographical  knowledge  increased, 
the  sources  from  which  they  could  be  obtained  became  more  nu- 
merous, while,  with  the  increase  of  physical  knowledge,  easier  and 
cheaper  methods  for  reducing  them  were  discovered,  and  the  ways 
in  which  they  could  be  rendered  beneficial  to  man  multiplied  pro- 
digiously. Not  a  day  passes  in  which  every  individual  of  all  civil- 
ized races  does  not  repeatedly  derive  benefit,  either  directly  or 
indirectly,  from  the  use  of  most  or  all  of  these  metals,  and  the 
amount  and  variety  of  their  use  by  any  people  is  an  infallible  index 
of  their  degree  of  civilization. 

Distribution. — Most  of  the  metals  or  their  ores  are 
widely  distributed  over  the  globe,  occurring  among  rocks 
of  various  geological  ages.  They  are  generally  most 
abundant  in  regions  of  highly  tilted,  disturbed,  or  meta- 
morphosed strata,  such  as  characterize  mountain  regions. 
This  is  partly  due  to  the  fact  that  the  enormous  erosion 
in  such  regions  has  exposed  a  greater  variety  of  forma- 
tions ;  but  it  seems  probable  that  many  of  the  metals  were 

P.  G.-9I. 


368  PHYSICAL  GEOGRAPHY. 

forced  up  from  below,  in  a  state  of  solution,  as  a  conse- 
quence of  the  upheaval  of  these  regions,  and,  upon  pre- 
cipitation in  the  fissures  of  the  dislocated  strata,  combined 
with  other  substances  present  to  form  the  ores  and  native 
alloys  of  the  metalliferous  veins  and  lodes. 

Iron,  the  most  abundant  and  useful  metal,  is  obtained  from 
several  kinds  of  ore — magnetic,  hematite,  limonite,  etc.  Iron  ore 
is  mined  and  reduced  in  every  civilized  country.  Most  of  that  re- 
duced in  the  United  States,  which  yields  about  one  fifth  of  the 
world's  supply,  comes  from  the  south  shore  of  Lake  Superior,  from 
the  Appalachian  region,  and  the  Ozarks ;  but  great  quantities  of  ore 
exist  throughout  the  west.  Copper  is  found  as  ore,  and  also  in  the 
metallic  state.  Copper  is  found  in  almost  all  regions  of  old  meta- 
morphic  rock.  The  richest  mines  in  the  world  are  in  northern 
Michigan,  and  in  Chile,  Spain,  and  Australia.  Tin,  zinc,  and  lead 
are  found  as  comparatively  easily  reducible  ores.  Tin  is  most  exten- 
sively mined  in  the  East  Indies,  Australia,  and  south-west  England, 
but  deposits  are  known  to  exist  in  Mexico,  and  in  California,  Alabama, 
and  elsewhere  in  the  United  States.  Zinc  is  mined  in  Illinois,  Mis- 
souri, Kansas,  New  Jersey,  Tennessee,  and  in  several  European 
countries,  notably  Germany  and  Belgium.  Lead  is  largely  produced 
in  the  Rocky  Mountain  region  in  the  reduction  of  silver  ore,  but  is 
mined  in  the  Ozarks,  Illinois,  England,  Germany,  and  Spain.  Gold 
in  the  metallic  state,  occurs  in  veins  of  quartz  in  metamorphic  rock, 
and  as  fine  grains  (gold  dust)  in  stream  deposits  composed  of  the 
eroded  and  disintegrated  debris  of  such  rock.  Traces  of  gold  are 
found  in  such  deposits  in  nearly  all  mountain  regions,  but  the  richest 
yet  worked  are  in  the  Sierra  Nevada  of  California,  the  Australian 
Alps,  and  the  mountains  of  south-east  Africa.  The  United  States 
supplies  about  one  third  of  the  world's  yield  of  gold.  Silver  is  some- 
times found  in  a  metallic  state,  but  generally  combined  with  sulphur 
as  an  ore.  It  is  specially  abundant  in  the  Cordilleras  of  North  and 
South  America.  Colorado  affords  about  one  half  the  silver  product 
of  the  United  States,  which  supplies  about  one  half  the  yield  of  the 
world.  Mercury  melts  at  a  temperature  of  about  370  below  zero, 
and  is  the  only  metal  that  is  liquid  at  ordinary  temperatures.  It 
usually  occurs  as  an  ore  called  cinnabar.  Mines  in  the  Coast  Range 
of  California  supply  about  one  half  the  world's  annual  yield.  Most 
of  the  rest  comes  from  the  mountains  east  of  the  Adriatic  Sea,  and 


MAN.  369 

from  Almaden,  Spain.  Antimony,  platinum,  and  nickel  are  the  only 
metals  of  comparatively  recent  discovery  that  have  been  largely  used 
in  the  arts.  The  ore  of  antimony  is  obtained  chiefly  in  the  East 
Indies,  but  is  found  in  both  Europe  and  North  America.  Platinum, 
like  gold,  is  found  in  minute  metallic  grains  in  alluvial  deposits. 
Three  fourths  of  the  world's  supply  comes  from  the  Ural  Mountains. 
Nickel  ore  in  minute  quantities  is  very  widely  distributed.  The 
mines  of  the  Sudbury  district  in  Canada,  furnish  the  world's  chief 
supply.     It  is  also  found  in  Saxony,  Sweden,  and  New  Caledonia. 

Various  other  minerals  besides  the  metals  are  largely 
collected  and  used  by  man.  Among  these  are  the  many 
kinds  of  building  stone,  clays  for  making  brick  and 
pottery,  marls  for  fertilizing  the  soil,  salt,  and  the  precious 
stones  or  gems,  used  both  in  the  arts  and  for  ornaments. 
But  the  mineral  whose  use  is  confined  most  exclusively  to 
civilized  man,  and  the  loss  of  which  would  affect  him 
most  seriously,  is  coal,  or  mineral  fuel. 

Coal  is  the  most  widely  distributed  and  the  cheapest 
source  of  great  and  easily  available  heat,  or  kinetic  energy, 
that  man  has  ever  discovered.  It  is  only  since  the  recog- 
nition of  its  great  thermal  value,  about  700  years  ago,  that 
iron  and  steel  have  been  manufactured  cheaply  and  in 
large  quantities,  while  the  rapid  development  of  all  kinds 
of  manufacturing,  which  followed  the  invention  of  the 
steam  engine  150  years  ago,  was  largely  due  to  the  rela- 
tive abundance  and  cheapness  of  mineral  fuel. 

Formation  of  Coal. — True  coal,  though  a  stony  sub- 
stance occurring  in  layers  interstratified  with  sedimentary 
rocks  of  various  geological  eras,  is  organic  matter.  It  is 
chiefly  the  metamorphosed  remains  of  a  swamp  vegetation 
which  flourished  on  the  earth's  surface  thousands  of  years 
ago.  As  such  vegetation  died  and  fell,  it  was  covered 
with  water,  and  thus  protected  from  the  atmosphere,  and 
consequently  from  rapid  decomposition  into  stable  com- 
pounds— carbonic  acid,   etc.     Thus,  a  thick  layer  of  or- 


37°  PHYSICAL    GEOGRAPHY. 

ganic  matter  accumulated  on  the  bottom  of  the  swamp, 
which,  when  more  deeply  submerged  by  a  gradual  sub- 
sidence of  the  region,  was  covered  and  buried  by  layers 
of  ordinary  inorganic  sediment.  When  subsequent  eleva- 
tion, or  the  accumulation  of  sediment,  brought  the  surface 
of  the  region  nearly  to  the  water  surface  again,  swamp 
vegetation  sprang  up,  and  another  incipient  coal  seam  was 
formed,  and  so  on.  As  the  weight  above  increased,  the 
buried  layers  of  organic  matter  gradually  became  com- 
pacted, while  the  increase  of  temperature  as  they  became 
more  deeply  buried  (page  186)  caused  the  complex  and 
unstable  protoplasmic  compounds,  rich  in  carbon,  gradu- 
ally to  break  up  and  form  simpler  compounds  in  which 
carbon  did  not  enter  so  largely,  such  as  water,  sulphuretted 
hydrogen,  carbonic  acid,  marsh  gas,  etc.  The  residue 
was  consequently  left  exceedingly  rich  in  carbon,  generally 
combined  to  a  greater  or  less  extent  with  hydrogen,  as 
bitumen.  This  unstable  residue  constitutes  coal,  and  the 
associated  hydro-carbons,  naphtha,  natural  gas,  petroleum, 
mineral  tar,   asphaltum,   etc. 

The  carboniferous  era  of  geological  time  marks  the  period  when 
the  bulk  of  the  plant  life  of  the  globe  had  developed  to  a  stage  suit- 
able for  the  growth  of  swamp  vegetation.  In  that  era  it  seems  to 
have  consisted  chiefly  of  gigantic  ferns,  cycads,  and  pines;  and 
such  vegetation  was  very  prevalent,  causing  by  far  the  thickest  and 
most  extensive  coal  deposits  yet  discovered.  Such  coal-forming 
vegetable  deposits  occurred,  however,  during  all  subsequent  ages, 
and  are  to-day  accumulating,  a.sfieatt  in  very  many  localities,  notably 
in  the  Dismal  Swamp  region  of  Virginia  and  North  Carolina ;  be- 
neath the  swampy  "trembling  prairies"  of  southern  Louisiana ;  in  the 
bogs  of  Ireland  ;  and  about  the  shores  of  the  Baltic.  The  transfor- 
mation of  vegetable  matter  into  true  bituminous  coal  is  an  ex- 
cessively slow  process.  It  seems  in  general  to  require  as  long  a 
time  as  from  the  Jurassic  period  to  the  present,  for  as  we  go  back 
among  successively  older  rocks  we  nnd  the  peat  deposits  of  the 
present  era  insensibly  passing  into  lignite,  or  brown  coal,  in  the 
tertiary  era,  which  becomes  more  and  more  thoroughly  carbonized 


MAN.  371 

as  we  proceed,  until,  in  the  Jurassic  period,  true  bituminous  coal 
occurs.  In  regions  that  have  been  subjected  to  exceptionally  great 
heat  or  pressure  the  process  has  been  hastened,  and  in  some  regions 
has  progressed  beyond  the  stage  of  bituminous  coal ;  thus,  where  the 
carboniferous  strata  in  north-eastern  Pennsylvania  are  most  ex- 
tremely plicated  and  contorted,  the  inclosed  seams  of  coal  have  lost 
much  of  their  bitumen,  and  have  been  compressed  into  the  hard, 
more  thoroughly  carbonized,  and  most  valuable  heating  coal — an- 
thracite. In  the  older  contorted  rocks  of  Rhode  Island  and  Mas- 
sachusetts, anthracite  coal  has  advanced  a  stage  further  and  lost 
much  of  its  value  as  fuel,  by  transformation  into  nearly  pure  carbon 
ox  graphite.  In  several  localities  in  the  West,  where  lava  dikes  have 
intersected  the  relatively  young  cretaceous  and  tertiary  strata,  the 
included  seams  of  lignite  are  found  to  have  been  transformed,  in 
the  vicinity  of  the  dikes,  into  true  bituminous  coal,  or  even  an- 
thracite, by  the  great  heat  of  the  lava. 

Distribution. — Deposits  of  coal,  near  enough  to  the 
earth's  surface  to  be  accessible,  are  widely  distributed. 
Fully  one  tenth  of  the  area  of  the  United  States,  and 
about  the  same  proportion  of  Europe,  are  known  to  be 
underlaid  by  workable  coal,  while  deposits  of  great  but 
unknown  extent  exist  in  China,  Canada,  Australia,  India, 
Chile,  Brazil,   and  elsewhere. 

In  the  eastern  part  of  the  United  States,  coal  of  carboniferous  age 
is  found,  under:  (1)  the  whole  Appalachian  plateau  from  northern 
Pennsylvania  to  central  Alabama — about  64,000  square  miles;  (2) 
the  central  part  of  southern  Michigan — about  7,000  square  miles ; 
(3)  the  southern  two  thirds  of  Illinois,  south-western  Indiana,  and 
western  Kentucky— about  47,000  square  miles  ;  and  (4)  from  central 
Iowa  southward  across  western  Missouri  and  Arkansas  and  eastern 
Nebraska,  Kansas,  and  Oklahoma  into  central  Texas — about 
99,000  square  miles.  The  first  of  these  coal-fields  is  by  far  the  most 
extensively  worked,  and  supplies  more  than  three  fourths  of  the 
yield  of  the  United  States,  while  practically  all  of  our  anthracite 
coal  comes  from  the  small  area  of  this  field  in  Pennsylvania.  True 
bituminous  coal  of  triassic  age  is  found  in  central  Virginia  and 
North  Carolina.  In  the  western  half  of  the  United  States  the  sur- 
face is  largely  composed  of  rocks  more  recent  than  the  Jurassic,  and 
the  extensive  coal  deposits  found  in  nearly  all  the  states  have,  in 


372  PHYSICAL    GEOGRAPHY. 

general,  advanced  only  to  the  stage  of  lignite.  This,  though  valu- 
able as  fuel,  and  closely  resembling  coal,  is  not  so  valuable  for 
some  manufacturing  purposes.  In  the  vicinity  of  dikes  in  this 
region,  as  before  mentioned,  and  in  the  regions  of  contorted  strata 
along  the  flanks  of  mountain  ranges,  where  the  great  erosion  has 
exposed  older  rocks,  true  bituminous  and  anthracite  coal  are  found. 
Over  500  million  tons  of  coal  are  used  annually  in  the  world. 
About  one  third  of  this  is  mined  in  the  United  States ;  over  one 
half  of  the  remainder  in  England ;  and  most  of  the  rest  in  conti- 
nental Europe — Germany  being  by  far  the  largest  producer. 

Conclusion. — Thus,  through  their  continued  use,  or 
exercise,  man's  mental  powers  have  gradually  increased. 
With  his  constantly  expanding  faculties,  his  observation 
of  nature  becomes  more  exact,  and  he  daily  recognizes 
more  clearly  the  dependence  of  her  manifold  phenomena 
upon  each  other.  In  numerous  instances  he  has  gained  a 
sufficiently  clear  comprehension  of  her  great  and  immuta- 
ble laws  to  invoke  their  aid  at  will  in  securing  results 
beneficial  to  himself.  By  incessant  exertion,  he  maintains 
his  limited  power  to  thus  direct  the  operation  of  these 
laws  into  such  channels  that  they  may  produce  about  him 
the  peculiar  and  artificial  environment  essential  to  civiliza- 
tion. But  the  observation  of  nature  involved  in  the 
attainment  of  high  civilization,  results  in  far  more  than  the 
development  of  man's  inventive  genius  and  the  improve- 
ment of  his  material  surroundings.  In  partially  revealing 
the  harmonious,  yet  marvelously  intricate  plan  on  which 
the  world  has  been  modeled,  it  teaches  him  of  the  utter 
insignificance  of  his  own  unaided  powers,  and  increases  his 
faith  in  and  reverence  for  the  Divine  Wisdom  which  de- 
vised and  which  maintains  it  all. 


INDEX 


Absorption  of  radiant  energy 21 

Adaptation,  of  organisms 318 

effect  of 327 

Adhesion 14 

Aerolites 38 

Affinity,  chemical 9>  x5 

Africa,  characteristic  life-forms  of.. . .     341 

surface  of *74 

Age,  of  mountains 253 

of  rocks 19° 

of  valleys 226 

Aggregation,  states  of 12 

Air,  capacity  for  vapor 66 

composition  of 55 

density  of 57 

humidity  of 67 

mechanical  cooling  of 69 

saturated 66 

weight  of 56 

Alloys,  metallic 367 

Amoeba 315 

Animals,  classification  of 321 

distribution  of 328 

domestic 366 

how  differ  from  plants 317 

vertebrate 323 

Antarctic  regions,  ice  of no,  121 

land  in 160 

size  of. no 

Anticyclones,  defined 95 

effect  on  weather 295 

Antitrade  winds,  defined 83 

Aqueous  rocks 183 

Archipelago,  Arctic 156 

Malay 156,  178 

Arctic  Ocean,  size,  etc no 

Artesian  wells 196 

Asia,  life-forms  of  Oriental  region 343 

monsoons  of 86 


PAGE 

Asia,  surface  of *7X 

Asphaltum i92»  37° 

Athermanous  bodies,  defined 21 

Atlantic  Ocean,  bottom  of. 114 

coast-line  of "i 

depth  of "i 

drainage  basin  of 207 

size  and  shape  of. 109 

temperature  sections 118,  141 

winter  winds  of 88 

Atmosphere,  color  of. 103 

defined 55 

density  of 57 

distribution  of  vapor  in 67 

electricity  of. 106 

heat  of 58 

height  of 57 

moisture  of 66 

pressure  of 56.  8t 

uses  of. 58.  59 

Atmospheric    pressure     at     tropics, 

equator,  and  poles 82 

Atolls 159 

Atoms,  defined 8 

Aurora  Polaris 108 

Australia,  characteristic  life-forms  of  339 

surface  of 177 

Autumnal  equinox,  the 5^ 


Bad  Lands 257 

Barometer,  the 56,  58 

Bayou 229 

Biological  regions 335 

Brain  of  man  and  animals 35I_3 

Breakers 124 

Breathing,  its  effect 316 

Breeze,  land  and  sea 89 

Buttes 259 

(373) 


374 


PHYSICAL   GEOGRAPHY. 


C 

PAGE 

Calderas,  how  formed 284 

Calms,  belts  of 83 

Campos,  pampas,  etc 330 

Canons,  formation  of. 222 

in  eastern  United  States 226 

of  Colorado  River 166,  222 

Capillary  attraction 14 

Carbonic  acid,  in  atmosphere 56 

in  sea-water "6 

in  spring- water 201 

Cataracts  and  cascades 224,  226 

Caverns,  formation  of 201 

Cells,  differentiation  of 316 

in  living  matter 314 

organisms  composed  of  single 315 

segmentation  of 315 

Centrifugal  force ....    44 

Chemical  affinity 15 

Chemical  heat,  how  produced 19 

Circles,  great 45 

small 4° 

Civilization,  ancient  Egyptian 354 

development  of 356,  365 

modern 367 

Cliffs,  formation  of 224,  258 

never  very  high 164 

Climate,  continental  and  oceanic 301 

defined 297 

effect  of  elevation 308 

"      "exposure 310 

"      "  land  or  water  surface 24,  299 

"      "  latitude 297 

"      "  mountain  ranges 311 

"      "  ocean  currents 304 

"      "  on  life-forms 328 

in  torrid  zone 306 

on  east  and  west  coasts 302 

Cloud  bursts 97 

Clouds,  color  of 104 

formation  of 69 

height  of 69 

kinds  of 70 

shape  of 71 

uses  of 71 

Coal 192,  369 

Cohesion,  denned 9,  11 

power  of 13 

Color,  of  clouds  and  snow 104 

theory  of 103 


PAGE 

Columnar  structure  of  rocks 185 

Combustion 16 

Comets,  described 37 

Compass,  directions  of  the 44 

variation  of  the 32 

Compound  substances 15 

Condensation,  effect  of 67 

Conduction,  of  electricity 33 

of  heat 20 

Conservation  of  energy,   defined 18 

Continental  climate 301 

Continental  islands,  defined 155 

distribution  of 156 

Continental  plateau,  described 151 

permanence  of 148,  347 

Continents,  characteristic  life-forms  of  338 

grand  divisions  of 155 

relative  areas  of 154 

Contraction  and  expansion  of  bodies  23,  25 

Convection  of  heat 20 

Coral  islands  and  reefs 159 

Coronas 106 

Corrasion 216,  218 

Crater,  volcanic 278,  284 

Crevasses,  glacial 236 

in  banks  of  streams 229 

Cryptogamic  plants 320 

Crystallization,  defined 12 

Currents,  velocity  in  streams 213 

(see  Ocean  currents.) 

Cyclones,  described 92 

effect  on  weather 293 

D 

Day  and  night,  cause  of 43 

length  of 5i»  298 

Decomposition 16 

Deltas 227 

Denudation 187 

Deposits,  of  geysers 286 

of  lakes 244 

of  sea *45 

of  springs  and  percolating  water. . .     202 

of  streams 218,  227 

Deserts,  defined 33° 

Development,  of  cell 3*5 

of  embryo ' 324 

of  land  life 348 

Development  theory 326 

applied 333.  336 


INDEX. 


375 


PAGE 

Dew,  definition  of 74 

formation  of 75 

Dew-point,  defined 75 

Diathermanous  bodies,  defined 21 

Digestion,  use  of. 318 

Dikes,  lava 185,  291 

Dip  of  rock  strata 189 

Direction  of  the  earth's  rotation 44 

Distribution  of  life 346 

effect  of  climate  on 328 

effect  of  isolation  on 333 

marine 349 

Drainage  basins,  oceanic  and  inland. .206-7 

of  stream  systems 211 

Drift,  glacial 236,  239,  240 

Dust,  in  atmosphere 56 

relation  to  mist  or  fog 69 

E 

Earth,  the,  axis  of 43 

composition  of  crust 180 

condition  of  interior 42 

density  of 41 

inclination  of  axis  of 49 

internal  temperature  of 41 

magnetism  of 30,  32 

poles  of 43 

position  among  planets 36 

orbit  of 48 

radiates  heat 59 

revolution  of 48 

rotation  of. 43 

shape  of.   38 

size  of. 39 

surface  of. 149 

Earthquakes,  causes  of 190,  264,  272 

effects  at  earth's  surface 272 

elastic  waves  of. 266 

energy  of 271 

epicentrum  of 269,  272 

frequency  of. 265 

in  United  States 265,  276 

origin  of 268 

propagation  of 269,  271 

sea  waves  caused  by 276 

sounds  caused  by 274 

Ecliptic,  plane  of 49 

Electrical  condition  of  matter 32 

Electric  spark,  defined 34 

Electricity 28 


PAGE 

Electricity,  atmospheric 106 

aurora  108 

generated  in  tornadoes 97 

generated  in  volcanic  eruptions. . . .  285 

lightning 106 

St.  Elmo's  fire 108 

thunder 107 

Elements,  chemical 7,  15 

in  earth's  crust 180 

Elevation,  continental,  region  of 150 

decrease  of  temperature  with 310 

mean,  of  land 161 

variation  of  life  w.ih 329 

Embryo,  development  of. 324 

Energy 17 

heat  and  light  forms  of. .18 

of  earthquakes 271 

of  volcanoes 285 

radiation  of 19 

Environment,  changes  in 327 

defined 319 

Equator,  defined 45 

the  thermal 64 

Equinoxes,  the 52 

Erosion,  a  cause  of  earthquakes 264 

amount  of 219 

curve  of. 220 

defined 181 

effect  of  corrasion  upon 218 

effect  on  land  surface 219,  256 

fantastic  forms  of 259 

in  folded  strata 259 

in  mountain  regions 249 

Estuaries 229 

Ether,  luminiferous 19 

Euro-Asia,  characteristic  life-forms  of  344 

surface  of. 171 

Evaporation,  cooling  effect  of 67 

defined 25 

from  streams 208 

influences  ocean  currents 136,  144 

variation  of 77 

Expansion  and  contraction  of  bodies  23,  25 

F 

Fata  Morgana 102 

Faulted  strata 189,  248,  252 

Fissures,  caused  by  earthquakes 273 

Fiords *29 

tixeu  stars.. as 


376 


PHYSICAL  GEOGRAPHY. 


PAGE 

Floods  in  streams 213,  215 

Fog,  explained 68 

Food,  use  of 317 

Forces  of  nature 9 

Forest  regions 330 

Fossils 183,  191 

order  of  appearance 325 

Fragmental  rocks 183 

Frost,  hoar 75 

on  hillsides 311 

Fumaroles 280 

G 

Gas,  defined 12 

Geological  time,  length  of 193 

Germ-cell 314 

development  of 315 

Geysers 286 

Glaciers,  distribution  of 231,  238 

effects  on  the  land 236,  239 

formation  of 231 

former  extent  of 237 

melting  of 234 

moraines  of 235 

movements  of 233 

size  of 233 

streams  from 237 

Grand  divisions  of  land 155 

average  elevation  of 161 

Graphite 371 

Gravitation,  defined 9 

effect  of  distance  on 10 

some  effects  of 11 

Great  Lakes,  the  formation  of. 240 

Gulf  Stream,  the 140,  142 

H 

Hail,  formation  of 74 

produced  by  tornadoes 97 

Halos 106 

Heat,  affects  size  of  bodies 22 

all  bodies  possess 19 

conduction  and  convection  of 20 

conveyed  by  ocean  currents 61,  140 

how  imparted  to  atmosphere 58 

latent 24,  61 

mechanical  equivalent  of 26 

nature  of 18 

radiation  of 19 

reflection,  transmission,  and  absorp- 
tion of ,,,,,. 31 


PAGE 

Heat,  specific 23, 60 

Hemispheres 45 

Heredity,  defined 318 

effect  of 327 

Highlands,  defined 161 

of  Africa 174 

of  Euro-Asia 171 

of  North  America 166 

of  South  America 169 

of  world 154,  179 

Hills,  defined 162 

Hoar-frost 75 

Horizon,   defined 39 

sun  appears  large  near 100 

sun  visible  below 100 

Humidity 67 

Hurricanes 94 

Hygrometer 68 

I 

Icebergs 119,  142 

Ice,  crystals i3>  72 

formation  of 25 

of  the  sea 118 

Igneous  rocks 185 

Impenetrability  of  matter 8 

Indestructibility  of  matter 8 

Indian  Ocean,  bottom  of 114 

coast-line  of 111 

currents  of  north 140 

depth  of MI 

drainage  basin  of 207 

size  and  shape  of. 109 

Induction,  electrical 34 

magnetic 29 

Inertia  (centrifugal  force) 44 

defined 8 

Intus-susception 314 

Islands,  continental 156 

coral 159 

life-forms  of  oceanic 336 

oceanic 157 

Isotherms,  defined 61 

of  United  States 309 

of  world 63,  65 

K 

Kinetic  energy 17 

Kuro  Siwo,  the 140 

L 

Laccolites 283 

Lagoons , 247 


INDEX. 


377 


PAGE 

Lakes,  color  of 245 

crescent-shaped 230 

distribution  of 246 

effect  on  climate 304 

effect  on  floods 245 

formation  of 241 

fresh  and  salt  water 242 

obliteration  of 244 

shape  of S46 

spring 201 

temperature  of 245 

the  Great,  of  United  States 240 

Lambert's  projection 53 

Land,  area  of 149 

effect  of  erosion  on 219,  256 

effect  of  glaciers  on 239 

effect  on  climate 60,  64,  299 

has  been  submerged 188 

height  of 112,  150,  161 

slips  or  slides 204 

surface  of. 161 

Languages,  resemblances  of 357 

Latent  heat 24 

amount  of . 61 

Latitude,  defined 46,  47 

effect  on  climate 297 

variation  of  life  with 328 

Lava,  composition  of. 180 

dikes 185,  282 

fluidity  of 280 

how  discharged 279 

laccolites 283 

streams 280 

Level  of  the  sea 149 

Life,  ancient  fossil 325 

classification  of 319 

distribution  of  land- 328,  331,  346 

forms  compared 324 

great  regions  of 335,  346 

higher  forms  of 315 

manifestation  in  matter 313 

marine 349 

simplest  forms  of. 315 

Light,  diffusion  of 27 

nature  of 18 

phenomena  of 100 

refraction  of 26 

selective  absorption  of 28 

Lightning 106 

Lignite 371 


PAGE 

Liquid,  defined 12 

Living  matter,  peculiarities  of 314 

Llanos,  pampas,  etc 330 

Longitude,  explained 46,  47 

Looming 102 

Lowlands,  defined 161 

of  Africa 177 

of  Euro-Asia 172 

of  North  America 169 

of  South  America 170 

of  world 179 

Luminous  bodies 19 

phenomena 100 

M 

Magnetic  storms 32 

Magnetism,  cause  of  earth's 32 

described 28 

of  earth 30 

Man,  ancestral  race  of. 356,  358 

ancestry  of 353 

compared  with  animals 350,  353 

in  savage  state 364 

prehistoric 354,  355 

races  of 358,  364 

Map  projections 53 

Marsupial  animals 323 

Mass,  explained 9 

Matter,  defined 7 

organic 314 

properties  of 8 

Meadows,  life-forms  of. 330 

Mechanical  cooling  and  heating  of  air      69 

Mechanical  equivalent  of  heat 26 

Mercator's  projection 53 

Meridians,  defined 45 

Mesas 257 

Metalloids 15 

Metals 15,  367 

distribution  of 367 

Metamorphic  rocks 185,  186 

Meteors 37,  58 

Mi  lcrals,  refined 180 

dissolved  in  lake  and  river  water. .     242 

"         in  sea-water 115 

"         in  spring-water 200 

of  great  use  to  man 367 

Mirage 102 

Mist,  explained 68 

Molecular  motion 18 


378 


PHYSICAL    GEOGRAPHY. 


PAGE 

Molecules,  denned 8 

motion  of 19 

Monotreme  animals 323 

Monsoons,  defined 84 

effect  of  in  Indian  Ocean 140 

of  Asia  and  Australia 86 

of  North  America 88 

Moon 37 

cause  of  tides 130 

Moraines,  defined 235 

in  United  States 239 

Mountains,  age  of 253 

defined 162 

erosion  of 249,  259 

granitic  crests  of 263 

influence  on  climate 311 

influence  on  winds 89 

of  Africa  and  Australia 176,  178 

of  America 166,  170 

of  Euro-Asia 171 

of  faulted  strata 252 

of  folded  strata 249,  261 

origin  of 255 

rate  of  upheaval 253 

.  stratified  rock  in 255 

structure  of 248 

table,  or  mesas 257 

Movements  of  earth's  crust 148,  190 

cause  earthquakes 264 

effect  distribution  of  life 338,  347 

fold  and  break  strata 188 

in  mountain  regions 253 

N 

Natural  gas 192,  370 

Nature,  forces  of 9 

laws  of 7 

Nebular  theory,  the 38 

Newfoundland  Banks 142 

Niagara,  cataract  of 225,  226 

Night,  cause  of 43 

length  of 51,  298 

Nitrogen,  in  air 55 

in  sea-water u6 

North   America,    characteristic    life- 
forms  of . .     344 

surface  of 166 

Nutrition 317 

O 

Ocean  currents,  causes  of 135 

deep 143 


PAGE 

Ocean  currents,  direction  of 136 

effect  on  climate 61,  304 

"        "   sea  temperature 140,143 

Gulf  Stream 140 

into  land-locked  seas 144 

surface 137 

Oceanic  climate 301 

drainage  basins 207 

Oceanic  islands 157 

life-forms  of 336 

Oceans,  boundaries  and  dimensions..  109 

Oozes,  on  sea-bottom 146 

Orbit,  dtfined 36 

of  earth 48 

of  moon 133 

Ores,  metallic 367 

Organic  matter,  defined 314 

Organisms,  cause  of  variety  of 327 

classification  of 319 

defined 314 

increasing  complexity  of. 324 

individuality  of 318,  319 

order  of  appearance  on  earth 325 

Organs,  defined 314 

development  of 316 

Oxidation 16 

Oxygen 15 

in  air 55 

in  life-processes 316-7 

in  sea-water. 116 

P 

Pacific  Ocean,  bottom  of 114 

coast-line  of in 

depth  of in 

drainage  basin  of 207 

size  and  shape  of 109 

temperature  sections  of 118,  141 

tides  of 133 

winter  winds  of 88 

Pampas,  steppes,  etc 330 

Parallels ....  45 

Peat 370 

Petrifactions 192 

Petroleum 192,  37° 

Phenogamic  plants 320 

Physical  Geography,  defined 7 

Plains,  defined 162 

rock  strata  in 248 

Planetoids 37 


INDEX. 


379 


PAGE 

Planets 35,  36 

origin  ol 38 

Plants,  classification  of 320 

cultivated 366 

differ  from  animals 317 

distribution  of 328 

how  they  feed 317 

Plateau,  continental 151 

defined 162 

formation  of 257 

submarine 114 

Polar  circles  and  tropics 50 

Polar  light,  the 108 

Polar  projection 54 

Pole,  north  and  south 44 

Pole  star 45 

Population  of  world 364 

Port,  establishment  of  the 132 

Potential  energy 17 

Prairies,  steppes,  etc 330 

Precipices,  never  high 164 

Precipitation,  distribution  and  amount  75 
(see  Rain-fall). 

Prehistoric  man ' 354-5 

Pressure,  atmospheric 56 

belts  of  high 82 

Probabilities,  weather 294-297 

Projections 53 

Proteids,  defined 313 

Protococcus 315 

Protoplasm,  composition  of 313 

how  made 317 

in  germ-cell 314 

motions  in 315 

R 

Races,  of  men 358 

tidal 129 

Radiation  of  energy 19 

Rainbow,  the 105 

Rain-fall,  defined 75 

effect  of  ocean  currents  on 306 

effect  on  vegetation 311,  330 

influence  of  mountains  on 312 

in  United  States 303,  307 

on  land  surface 77 

proportion  discharged  by  streams..     208 

winter  and  summer 302 

Rain,  formation  of 71 

in  cyclones 94,  295 


PAGE 

Rain,  in  equatorial  calms 136 

in  tornadoes 97 

uses  of 72 

Rain-water 71 

Rapids 224,  226 

Reefs,  coral 159 

Reflection,  of  radiant  energy 21 

total 27 

Refraction,  defined 26 

displacement  of  sun  and  moon  by.  100 

Mirage,  Looming,  etc 102 

Relative  humidity 67 

Respiration 316 

Revolution  of  the  earth 48 

Rivers,  of  Great  Plains 224 

relative  size  ot 210 

(see  Streams). 

Rocks,  classification  by  age 193 

columnar  structure  of 185 

composition  of 180 

corrasion  of 216 

disintegration  of 181 

effect  of  glaciers  on 236 

erratic 237 

metamorphic 185 

permeability  of 195 

plastic  under  pressure 42,  151,  164 

position  in  mountains 249,  254,  260 

primitive 187 

solution  of 181 

stratified 183 

unstratified 184 

weathering  of 181 

Rock-tables 235 

Rotation  of  the  earth 43 

influences  currents 136 

influences  winds 79 

S 

Saint  Elmo's  fire 108 

Salt-water  lakes 242 

Salt  water  of  sea 114,  243 

Sand-bars  in  streams 230 

Saturated  air,  defined 66 

Sea,  bottom  of 114,  145,  148 

continuity  of no 

currents,  affect  temperature 140,  143 

"          causes  of 135 

"          deep 142 

"          to  arms  of 144 


38o 


PHYSICAL    GEOGRAPHY. 


PAGE 

Sea,  currents,  surface 137 

deposits  of 145 

depth  of in,  112 

earthquake  waves  of 276 

extent  of 109 

ice  of 118 

level  of  the 149 

saltness  of 114,  116,  243 

temperature  of 117,  140,  143 

tides  of 125 

waves  of 122 

Seasons,  the , .       52 

wet  and  dry 306 

Sea-water,  composition  of. 114,  243 

Sedimentary  rocks,  formation  of 183 

in  mountain  regions 255 

Sediment,  forms  rocks 183 

in  glacial  streams 237 

in  lakes 244 

in  streams 218 

Selective  absorption 28,  104 

Sierra,  definition  of 263 

Simooms 93 

Simple  substances 7,  15 

Sink-holes 201 

Sky,  color  of 103 

Sleet,  origin  of 72 

Slopes,  of  stream-beds 21a,  220 

of  valley  sides 221 

influence  on  climate 310 

steepness  of. 163 

Snow,  color  of 104 

formation  of. 72 

uses  of. 74 

Snow-flakes,  shape  of 72 

Snow-line 73,  310 

Snow-storms 73 

Soil,  defined 181 

Solar  spectrum 28 

Solar  system 35 

origin  of 38 

Solfatara 286 

Solid,  defined 12 

Solstices 52 

Solution  of  rocky  matter 181 

in  lakes 242 

in  sea 114,  243 

in  springs 200,  202 

in  streams 216,  219 

Sounds,  produced  by  earthquakes. . .     274 


PAGE 

South    America,   characteristic    life- 
forms  of 340 

surface  of. 169 

Specific  gravity 10 

Specific  heat,  explained 23 

of  land  and  water  surfaces 60 

Specter  of  the  Brocken 102 

Spectrum,  the  solar 28 

Speed,  of  earthquake  propagation. . .  269 

of  earth's  revolution 48 

of  earth's  rotation 44 

of  light 20 

of  lightning 107 

of  stream  currents 213 

of  tidal  waves 127 

of  water  waves 123 

Spheroid 39,  44 

Spring  lakes 201 

Springs,  deep  seated 196 

deposits  of. 202 

effect  of  earthquakes  on 273 

hot 198,  286 

intermittent 196 

mineral 200 

surface 195 

temperature  of 198 

uses  of 198 

Stalactite 203 

Stalagmite 203 

Stars,  fixed 35 

Steam,  in  geysers 286 

in  volcanoes 279,  292 

latent  heat  of 25 

Steppes,  prairies,  etc 330 

Storms,  magnetic 32 

paths  of 91 

wind 9° 

zones  of 94 

Stratified  rocks 183 

disturbed  and  faulted 188 

position  in  mountain  regions.  ...249,  254 

relative  age  of 192 

unconformable 19° 

Streams,  alluvial  bottoms  of 227 

cataracts  and  cascades  of 224 

channels  of 230 

corrasion  of  bed 216 

defined 205 

deltas  of 227 

Streams,  discharge  of 208,  210 


INDEX. 


381 


PAGE 

Streams,  floods  of 214,  245 

glacial 237 

great  age  of 226 

material  transported  by 219 

rapids 226 

sinuosities  in  course 229 

size  of tic- 
slope  of. 212 

speed  of 213 

system  of 206 

valleys  formed  by 219 

volume  of. 213 

Striae 236 

Sun 36 

distance  of 48 

effect  on  streams 78 

effect  on  tides 130,  131 

effect  on  vegetation 317 

effect  on  winds 78 

heat,  imparted  to  atmosphere 58 

heat,  varies  with  latitude 49,  297 

source  of  heat  (energy) 58 

"Sun  drawing  up  water  " 102 

Sunsets,  red,  of  1883 104 

Surface,  movements  of  sea- 122 

of  Africa 174 

of  Australia 177 

of  earth 149 

of  Euro-Asia 171 

of  land 161 

of  North  America 166 

of  sea-bottom 144 

of  South  America 169 

Suspension,  rocky  matter  in.. 217,  219,  245 

T 

Talus 164,  224 

Temperate  zones,  weather  in 294 

Temperature,  abnormal 305 

annual  range  of 298 

decreases  with  elevation 59,  310 

effect  of  currents  on  sea- 140 

increases  with  elevation 311 

mean  annual  in  United  States 309 

measure  of 22 

of  atmosphere 59,  310 

of  deep  sea 117,  u8,  142 

of  earth's  interior 41 

of  lakes 245 

of  land  and  water 60,  64 


PAGE 

Temperature,  of  northern  hemisphere  61 

of  sea-surface 117,  118 

of  southern  hemisphere 62 

of  springs 198 

of  valleys  in  winter 311 

relation  to  specific  heat 23 

Tenacity  of  various  substances 13 

Terraces  of  glacial  drift 240 

Thermal  equator,  the 64 

Thermometer,  described 22 

wet  and  dry  bulb 68 

Thunder 107 

Thunder-storms 97 

Tidal  currents 126,  227 

waves 127 

Tides 125 

cause  of 130 

diurnal  inequality  of 133 

duration  of 129 

"  establishment  of  the  port  " 132 

height  of 128 

in  lakes  and  seas 134 

on  Pacific  and  Gulf  coasts 133 

races  caused  by 129 

spring  and  neap 132 

the  "  bore" 129 

Tornadoes 96 

frequency  in  United  States 99 

Torrid  zone,  weather  in 293 

Trade  winds,  the 83 

Transparent  bodies,  defined 21 

Transportation  of  sediment 216,  219 

Tropical  belts  of  high  pressure 82 

Tropics  and  polar  circles 50 

Tundras 329 

Twilight 101 

Typhoons 94 

U 

Unconformable  strata 190 

Unstratified  rock 184,  185 

V 

Valleys 163 

age  of 226 

buried  in  drift 240 

canoe-shaped 261 

formation  of 219,  224 

frosts  in 311 


3^2 


PHYSICAL    GEOGRAPHY. 


PAGE 

Valleys,  glaciated 236 

sides  of 221 

Vapor,  amount  in  atmosphere 55 

described 66 

distribution  in  atmosphere 67,  77 

humidity  of  air 67 

Variation  of  the  compass 32 

Vegetation  regions 331,  332 

Velocity  (see  Speed). 

Vernal  equinox,  the 52 

Vertebrate  animals 323 

Volcanic  necks 291 

Volcanoes,  activity  of 278,  286 

calderas  of 284 

causes  of  action 291 

cones  of 281 

craters  of 278,  284 

distribution  of ># 179,  288 

eruptions  of 40,  278,  284 

materials  discharged  from 279 

mud 287 

W 

Water,  freezing  of. 13,  25 

latent  heat  of 25 

maximum  density  of 25 

movements  in  waves 122,  126 

specific  heat  of 23,  26 

surface  affects  climate 24,  60,  64,  299 

Water-gaps 226,  262 

Water-shed 206 

Water-spouts 99 

Waves 122,  127 

elastic 266 

force  of 124 

from  submarine  earthquakes 276 

size  and  speed  of 123,  127 

tidal  waves 127 


PAGE 

Weather 293 

effect  on  rock 181,  222 

Weight,  defined 9 

of  air 56 

Wells,  artesian 196 

surface 195 

White-caps 124 

White  squalls 99 

Winds 78 

anticyclones 95 

antitrade 83 

cause  of 78 

classes  of 81 

cyclones 93 

cyclonic  or  storm 90 

diurnal,  in  valleys 89 

dust  whirlwinds 92 

effect  earth's  rotation  on  the 79 

effect  on  ocean  currents 137 

force  or  speed  of 81,  90 

hurricanes 94 

land  and  sea  breezes 89 

monsoons 84 

of  oceans  in  winter 88 

of  world 85,  87 

simoons 93 

spiral  whirl  of. 80 

tornadoes 96 

trade 83 

typhoons 94 

white  squalls 99 

Winter  solstice 52 

Y 

Yeast 315,  320 

Z 

Zones  of  the  earth 49 


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